Why both teams made the trade?
For the Bulls, in their current situation, they have quite a few wing players on the roster. Jimmy Butler and Dwayne Wade, of course, holding down starting spots, and Doug McDermott as another proven wing on the roster. Taj Gibson, Nikola Mirotic, and Bobby Portis are also likely to hold down most minutes at PF. That meant that Snell was, at best, left as the 4th wing on the roster, and given that Butler and Wade are likely to be split up quite a bit, there is good reason to believe that Snell would be left with not a ton of time.
With the Bucks, MCW had been a disappointment by any stretch, but with the emergence of Gianni's playing point guard and then the signing of Delly this past offseason there did not appear to be a lot of room for MCW to get minutes either. On top of this, the Bucks have lost Khris Middleton for at least a large portion of the season to injury, leaving a fairly large hole on the wing for them, and also left them in fairly desperate need of someone who could shoot a little.
The Bulls were very thin at the backup PG spot, however, with several unproven players like Jerian Grant and Spencer Dinwiddie competing for minutes. The Bucks were thin on the wing especially for shooters, and MCW did not have a clear spot on the team. Snell had been largely a disappointment for the Bulls, just as MCW has been largely a disappointment for the Bucks. There is a good chance both players are just not going to be any good, but both teams decided that they had a need where they could give the other guy a legitimate chance. With the Bulls MCW (should) be firmly in place as the backup PG, and Snell will get a chance to see plenty of playing time with the Bucks. Essentially, both teams figured that if they are going to take a chance on a young guy who might stink, they may as well get a guy that if he ends up being any good he will actually help the team more.
What does it mean for the Bulls?
MCW will take on the role of backup PG behind Rajon Rondo, and hopefully will give some amount of stability to their bench. If nothing else, MCW has shown at least some ability to get into the paint and do some good once he gets there. The thing that makes less sense about this is that with Rondo, Wade, and Butler, one would expect that one of those 3 would be on the floor at all times, and all 3 of those guys can do the same thing but much better. As such there did not seem to be too gaping of a hole in the Bulls' bench as far as scoring creation.
HOWEVA, given that he is likely to spend time playing alongside McDermott, Mirotic, and Portis with bench mobs, there is a decent chance MCW will actually get to play with more shooting around him than he has in his career before, and could end up being a decent fit playing PG in a spacer lineups and getting to attack the create offense from there. On the other hand, though, maybe Hoiberg will not put him with shooters and he will be plagued by having no space to operate, and it also may be that even with shooters around him he can't shoot enough or pass well enough to really do much good.
What about beyond this year?
This is the one area where this bet makes the most sense. The Bulls are (probably) trying to rebuild without bottoming out, essentially hoping that Wade and Rondo can be placeholders for the next couple of years while they build a team up for Jimmy Butler to lead. If MCW could actually put it together and become a viable NBA player, he is more on Jimmy Butler's timeline and could in theory be a part of the team after Wade and Rondo are gone, whereas even if Snell had ended up becoming a viable NBA player, Butler is the same sort of player and he is a star, and McDermott also is young and looks to have solidified himself as having some role in the future.
Essentially, if MCW figures it out, he could fill a potentially massive hole on the team, whereas if Snell had figured it out he would likely still be coming off the bench or, at best, losing lots of minutes to McDermott. Once again, the Bulls basically decided that if they have to hope a guy fills out, it may as well be a guy who could help the team more.
What does it mean for the Bucks?
Snell could honestly get a chance to start at SG out of the gate to give the Bucks a starting lineup of Delly/Snell/Giannis/Parker/Whichever center they decide to start (Monroe, Plumlee, Henson). Since with MCW was gone, Delly figures to not be filling in at SG alongside another “true” (aka not Giannis) PG as much as previously thought, and the only other guys who were figuring to get minutes at SG were Rashad Vaughn and Jason Terry. I'd guess that Vaughn will start, but Snell will have a chance to win it. At the very least, Snell will be getting consistent minutes off the bench.
Snell brings yet another long armed and athletic player to the Bucks, and although he has never lived up to his potential on defense he could, in theory, be another cog in a potentially terrifying defensive team. And on offense Snell has not shown a lot of ability, but he has been a fairly proven shooter, shooting 32%, 37%, and 36% from deep over his first 3 seasons. Given that the Bucks have plenty of other guys who can handle the ball and create offense the Bucks will not be asking him to do anything other than spot up on offense so it should be a good fit there.
What about beyond this year?
A kind of similar idea as MCW with the Bulls here, in that even if MCW put it together, he was topping out as a 6th man sort of player with this Bucks team. Whereas if Snell puts it together he could be yet another long armed terror on defense on the team, but also is a capable shooter which is a bit of a problem for the Bucks right now. Although I think that this move is more about this year for the Bucks than it is for the Bulls, because in the future Kris Middleton will return. HOWEVA, if the Bucks go all in on Point Giannis, a future starting lineup of Giannis/Middleton/Snell/Parker/Plumlee could very well happen.
So who won this trade?
I wouldn't say either team really “won” the trade in a meaningful way. Both guys have been pretty poor so far, and there is a very good chance that both of them end up staying that way and the only reason that the trade is worth noting is because of the spots they were both drafted. That said, I think that it is kind of a tie because, on one hand, I think MCW is probably a better player than Snell and more likely to be better in the future. That said, I think Snell is (theoretically) a better fit with the Bucks than MCW is with the Bulls. So if they both figure it out to some extent I think MCW will be a better player but Snell will be a better fit. In the end, though, the winner of this trade is likely to be decided by if one of these guys can actually figure it out and become viable NBA players.