People have already hit this nail on the head, but it's economy of practice. You can shoot all day long for $40, whereas $40 in .223 only buys you about 100 rounds.
1000+ rounds of 22LR in one day is a heck of alot of practice time. Learning controls, developing muscle memory, marksmanship skills and instincts. Transferring to your main rifle only requires you to learn recoil management which isn't much.