@Keiomestre - My game is in English. I don't know actually what it does because I didn't select it. At the time when I saw it, all the band members lived in the same household and I could just make them rehearse when I wanted to, so I didn't see a need in it.
According to what @GraceyManor posted, I imagine the option would appear when the band members (who aren't a part of the active household) are either on a lot where instruments are or, if on my active Sim's home lot my Sim would have to be playing their instrument. I'm guessing its function would be like the option that asks Sims to join in their jam session, only they would be called to rehearse. Late Night is currently uninstalled from my game so I won't get the opportunity to test this to see if it works or not.
Oh, it's not a Showtime related option because I only had Late Night installed when I saw it.