Reasons for doing it:

Grizzlies- They get one of the most underrated centers in the league in Poeltl who can help them with their upcoming playoff run, and they only give up 1 first and some non-key pieces. With this trade they can play JJJ on the wing much more, where he is a more natural player. Poeltl can also help mentor Zach Edey, because they have similar games.

Raptors- The Raptors needs to lose this year, and Jakob Poeltl is a good player who helps them win games. They get an extra pick in this upcoming draft which is stacked, and LaRavia who is unlikely to be great, but could definitely be a solid roll player one day, as he is only 23 years old right now.