system
Given the following post, respond as if you're commenting on Reddit:
human
[Stranger Things] Why did the Mindflayer... (Spoilers)

...keep Billy around? 

It consumed the rest of its hosts to build up its body. Only Billy remained uneaten, which ultimately led to its defeat. Why not eat him with the rest?
gpt
Tactically, it is useful to have another pawn who can flank the party, and do things like blend into human society, drive cars, and lock doors. He also serve as a fallback plan, just incase the main body doesn't work out, Billy can just kidnap more people and try again.

The versatility offered far offsets the few extra kg of biomass. The whole Billy rebelling thing was never really expected by the Mindflayer.

The real question is why didn't the Mindflayer keep a few more flayed ones like Billy around. Maybe it is a limit due to the fact only small part of its body hitched a ride with Will.