Why have saddlebags on a cruiser?
For me the answer is my hat.  When I stop riding I uplock the left bag and remove The Hat and my reading glasses.  Remove helmet (A Daytona Skull Cap) and place helmet, gloves, and riding glasses in saddlebag.  Lock bag.  Don't have to carry the helmet and don't worry about leaving it sitting on the bike unattended.

The bags are also handy for shopping trips.  I can fit a ten pound bag of potatos in one bag.