In the case of a blowout, it definitely makes sense to use a position player instead of tiring out an arm in the bullpen. Why waste a reliever when you're down by 10 in the later innings?
Well, for the most part, they keep it on the slower side and lob the ball to the catchers to preserve their arms.
There's also the risk of a line drive coming back at them hot off the bat and hitting them. That risk goes for anyone on the mound, but pitchers are slightly more accustomed to that possibility.
Here is the clip from yesterday's game. This 35.1 MPH Frank Schwindel pitch is the slowest (known/measured) MLB pitch to ever get hit for a home run!