Offensively, you are inflicting healing immunity, which is the focus of this attack. Additionally, you are calling a targeted assist, cleansing their debuffs, and granting foresight. There’s a lot going on in this attack. It’s more than just a cleanse.
By choosing BB-8, you get an additional attack when he assists and calls Rey to assist again. If not, it would be a waste of his zeta, and people would be complaining about that. I would hope that the game would prioritize the use of zeta abilities, because they are a big investment.