Reasons for doing it:

Heat- Jimmy gets to go to a contender like he wants, they get back 2 good players who can contribute right away, along with a 2031 1st which will likely be a good pick. They trade away Terry Rozier who is not having a great season so it's not a huge loss. They do get worse with this trade, but there it's likely the best offer they get and they just try to minimize losses with this trade.

Nuggets- The Nuggets are desperate right now, currently 4th in the West with the best player in the world, they need to capitalize. Jimmy gives them an elite 2nd option, especially when the playoffs roll around. He will also motivate the guys cuz we all know Jokic ain't doin any of that. They only have to give up 1 first (that's all they can give) along with some young pieces, but they aren't too valuable.

Wizards- They trade their best player to tank even harder for Cooper Flagg or Dylan Harper, they can likely trade Rozier for a bit more draft capital, they get some solid young players that could be good roll players one day. They would be a bit upset to not get a 1st for Kuzma, but this is a good offer and they would take it.