Their purpose is long overdue. The match-ups are over. Big Mom's already defeated. We're at Wano's boss battle. And when Onigashima lands, Oda must deject of them somehow.
My options:
Option A: They remind the Straw Hats who they're still dealing with but flee like Gato's henchmen from Naruto at the end of the Land Of Waves Arc for reasons I don't feel like listing.
Option B: Kizaru and the Marines show up and clean house with them.
Option C: Not sure how Oda would make sense of this, but after their recovery, Big Mom promises to leave with her crew if Luffy beats her one-on-one.
Option D: Something about O-Lin.
Option E: A rift among the Big Mom Pirates.
Option F: Anything else (can include any combination of the other options).
Side Note
Oda could've at least had Smoothie, Compote, and Daifuku tag along with Perospero to Onigashima. Shachi and Penguin team up against Compote, Heat and Wire team up against Daifuku, Bepo goes Sulong to beat Perospero, and Yamato handles Smoothie. Meanwhile, Snack and the other Charlottes stand guard for Queen Mama Chanter.