Some professional golfers relish playing the senior tour( see John Daly)
Others probably could care less.
Personally speaking,I would think the decision
to play on the Senior Tour might have something to do,more or less,with one's success on the regular PGA Tour.
For example,even healthy,I don't see Tiger playing the Senior Tour all though he might have some success.
He would really have nothing to gain by doing.
But for some of the older tour players,especially those missing a Major on their resume,it could be an opportunity to claim that elusive Major Title.
Besides,having to wait till your 50 is a long road.