The Spurs are looking to trade into a second lottery pick, and I'm pretty sure the goal is to get Wemby's teammate Bilal Coublibaly. Assuming he's there at 11, the Spurs snatch him, to pair him with Wemby by giving up a couple of the future picks they picked up from Toronto and Chicago (the Toronto one is top 6 protected from the Poeltl deal, and the Chicago one is top 10 protected from the Derozan deal).
From the Magic perspective, they are probably looking to turn the corner this year, and they own two lottery picks. They probably don't want two more rookies to develop, on a roster already stuffed with young guys. This deal kicks the can down the road, giving extra picks in future years, and dealing the 11th pick this year also leaves more cap room for their planned spending in free agency.
Who says no?