Like a quick wave in real-life, Tandem waves are multi-purpose. Use it to greet your teammates, like a "Hello!" or "Good morning." Or use wave to let them know you're free to talk. If a teammate waves at you and you aren't free at the moment, you can do a friendly wave back and catch up later.
Here's another way to use wave: If you're in a meeting and realize it could be nice to include a teammate, but isn't critical, wave at them to let them know they're invited to listen in.