Why does my 21 month old baby wake up in the middle of the night to have something to drink?

Sleep is a learned. Sleep is a learned behavior, she is waking because she is used to eating in the middle of the night. You have two options one is to just let her cry it out after a few nights she will no longer wake. You can give her only water in decreaseing increments and hope that she stops waking because water is quite a feeding.
Break the habit. It is likely that you inadventently trained this habit. All people wake up several times at night and learn to fall back to sleep. Parents go to check on them and give them some interaction which they then associate with the ritual of going back to sleep. You need to break the habit by cold turkey = not going in to child at all or staged =go in don't give drink x a few nights then don't go at all.