Could be that designer A is better at inventive (brand new) parts, and designer B is better at reproductive (updates based on old designs). Then there is trying to find better designers so one or more is a trial designer.
I've also heard that some designers appear to be better at grip AMD others at aero, so having multiples for that could also be advantageous.
Workload I don't yet have an opinion on whether it is better to do just a few designs per designer or if one can do it all without problems.
Morale AMD motivation also effect staff members, so a bad run could hurt that, so you hire a few new staff with better morale to bridge the gap while your old staff get themselves back to being happy.
There's plenty of reasons, plus only Frank knows for sure the real answers, the rest of us have to figure it out