The part that other comments left off is salary matching. Yes, the expiring contract is good because it gets the player off your books, but if you don't want the player they why do they get included in the trade at all?

When you do a trade in the NBA, if you're over the salary cap (which is basically every team that's trying to win), the salaries going out and coming in have to be close to each other. Draft picks count as $0 for the purposes of matching.

So, if you want to trade a player and get a draft pick back in compensation, then you have to get the other team to throw in a player whose salary is close to what you're sending out. But, since that player is only being included to make the salaries match, by definition it's someone that you don't actually want. The best possible scenario is a player who's getting paid enough this year to make salaries match but whose contract is expiring so that you're not committed to them beyond the end of the season.

Remember, you didn't want this player, they're only part of the trade to make the salary matching work for what you really wanted to do - trade a player for a draft pick.

It's not always player-for-draft pick, there are also player-for-player scenarios where this happens, but the reason is always the same (salary matching).