It also gives a lot of young pitchers a taste of the majors, De Los Santos, Suarez, etc.
That helps in player development (gee, they can hit a 95 MPH FB in the Show!), and getting them over the jitters next time they come up.
And of course, a taste of the "show" is a great motivator:
"Yeah, I was in the show. I was in the show for 21 days once - the 21 greatest days of my life. You know, you never handle your luggage in the show, somebody else carries your bags. It was great. You hit white balls for batting practice, the ballparks are like cathedrals, the hotels all have room service, and the women all have long legs and brains."