Three benefits that are tough to argue against:
- Technique Practice : Practicing good technique under time pressure is a skill that has a high ROI in slower OTB games that may often get into a time-crunch.
- Opening Practice : If you're shopping/trying out new lines, blitzing 10-20 games makes more sense than playing one slow serious game which won't let you explore an opening all that much.
-Tactics Practice: Working on puzzles where you KNOW that there's a winning move is one thing ... blitz tends to give you a good assessment of how far your tactical training has really taken you.
The title Say's it all.