Until we get an official continuation of Barris Offee's story, my headcanon is that Palpatine kept her as a backup apprentice in case Anakin refused to join him : r/legostarwars
Barriss Offee, sw0909 – Atlanta Brick Co
LEGO IDEAS - Star Wars the Clone Wars Barriss' Treachery
LEGO IDEAS - Star Wars the Clone Wars: Barriss Offee V.S Anakin Skywalker
LEGO Star Wars The Skywalker Saga Barriss Offee Unlock and Gameplay - YouTube