Does a sub-panel require its own local main breaker?
It's a convenience, but:

The breaker capability is unnecessary (and you can't make it trip locally first). However most breakers are actually combo devices, breaker+shutoff, breaker+GFCI, etc.  Those auxiliary functions are useful. 

In an outbuilding, the shutoff switch is mandatory.  In other locations like hot tub, a GFCI may be mandatory.  You would use main breaker panels there to obtain that auxiliary function.