The 223 shoots very flat, and is cheap to shoot with factory loads, or to reload for. They are just plain fun to shoot. They also give you a really good starting point to start Extending your comfort zone with Longer Range Shooting.
Then we have varmint hunting. The 223 is a great choice with fast expanding bullets.
Then we have very low recoil. Good for Women and Kids, heck it's even good for hairy chested men. No harm getting in trigger time with something that does not recoil hard.
Ammo is very plentiful, and cheap.
Reloading components are available every where. I have a couple 5 gallon bucket of brass just from picking up a little at the range now and again.
I am sure I missed a few other good points.

EDITED: It would be nice to read a well detailed post on a Custom 223 Barrel with a faster twist, and the results with heavier bullets.

Whats Not To Love!!

Bob