After trading away Brandon Knight, the Bucks needed to grab another guard who could shoot. They have Michael Carter-Williams who can facilitate, but Grievis Vasquez can give them shooting at the point guard spot off the bench. They might even need him to take over the starting role if they decide to trade MCW like some rumors have mentioned.
Either way, this seems like a good trade for both sides. The Raptors get a little insurance in the future with some picks, and the Bucks get someone who can come in and help out a playoff team. The interesting part is whether the Bucks decide to trade for more after this one.