Imagine you have a reference plane for which multiple other planes need a flush constraint.
Picking the same reference plane again and again for each constraint can be tedious and time consuming. Chance of error is bigger as well.
Give an option to lock the first selection, select a second plane, click apply, and the next constraint already has the first selection of the previous constraint.
You guessed it right, I'm indeed the master of excellent suggestions. Flowers, chocolates and presents of luxury in my inbox please.