Pumping settings?

Okay so my pump (Nature's bond) has 2 settings. It automatically starts on stimulation mode, then beeps after 2 minutes to let you know to switch to expression mode. Do I HAVE to switch? Or can I stay in stimulation mode? I tend to produce more in stimulation mode than I ever have in expression mode. Is there a reason you switch? Idk about this stuff. I've always switched just because directions said to. But I'm a FTM and don't know these things.