Many reasons for this:
Cost
Variety
Proximity
I've stayed at the Contemp for 2 nights and did two straight days at MK. (Basically walked right over). Then checked into Beach Club and hit EPCOT - DHS for another couple of nights. Don't deal with parking lots, bus transportation, or waste a lot of time waiting on such. Plus it's quick to hit the room and recharge and then be right back at it. Everybody has got their thing. I don't do that often, but I still do it from time to time.
It's not really a bother, For a 4 or 5 night trip you can pack pretty light. So either let Disney transfer your bags or preferably just drop them off with the bell service at the new resort. It doesn't really impact anything. If I left the Contemp at 8:30am and left my bags at Beach Club by 8:45am - (I've already checked in via the app) - then simply walk over to International Gateway to be there by 9am. By usually no later than 1pm - I've got a text about my new room.