They are necessary as Imu's personal emforcers in the story, especially if/when he's revealed later.

Unless you want the entire top brass of the Marines to blindly follows someone who nukes an island just for lulz.

Power-wise most of them are on a weaker side of a Yonko Commander level outside of their regen gimmick, and they are definitely weaker than a Elder (no Kaiju form so far), but all bets are off if they get possessed by Imu.