I would suspect to prevent elbow from hyper extending and that related injury.
This would be correct, and if you've ever hyperextended an elbow, you'll appreciate good elbow pads. I have, I used to wrestle in college. No elbow pads there and lots of people trying to bend your elbow in the other direction.
With that being said, 3 piece are going to feel different than 2 piece elbows. The more protective, the more bulky and restrictive the pad is going to feel. Some pads break in better than others. Some break down faster than others (that was my problem with CLs. Not the pad itself, but the neoprene strapping didn't hold up for me very well.) Getting the right size is also important, lots of guys are wearing Jofas that are too large because that's what sizes are easier to find. The problem is, most of us aren't quite built like the professional hockey players who wear all those size 6s.... Hence the complaints of slippage, etc.
The advice about trying on many is good, the problem is that what feels good in the store sometimes doesn't translate to feeling good on the ice. That's when you find that they bind up with your shoulder pads, slip when sweaty, etc. Spending $45 on 9Ks might be a less painful hit in the wallet to know if you like Jofa/Reebok styled elbows than spending the going rate for Pro 7Ks. Lots of other good suggestions here too.
