San Antonio has a roster spot not being utilized in any way, shape, or form, other than to allow Bismack Biyombo to hang out with some cool guys making a run through the Western Conference. He's a veteran in this league, so it's not like he's going to develop into a key role player the team can lean on later, but they could have been developing Maxime Reynaud for that very purpose.
Biyombo has played 51 seconds total over the last 11 games. He's also only seen the floor in 11 total contests this season. When the team picked him up, it was seen as insurance for Victor Wembanyama and Luke Kornet, but they also hoped to actually use him from time to time in certain matchups.