In some cases, it could be less output, or lower quality. Money (or other explicit rewards) changes the psychology of doing work vs. volunteering it. It adds pressure, makes the work feel like a job instead of a hobby, and can lead the person doing the work to choke on deadlines, slack off, look for shortcuts, or even give up entirely.
Of course, people providing valuable services to a for-profit business should be fairly compensated, and companies shouldn’t be trying to exploit volunteers.
But monetary “incentives” are in general quite a terrible system for eliciting good work.