I remember when I was a kid, and I had just gotten hit by an enemy before a loop and couldn't do it right way, thinking "duuude, why doesn't Sonic just jumps sideways?"
You could do the loop anyway, but if you don't have enough momentum just jump to the other side. It's nice.
Though I know that it is now a "trademark" of the game you having to really go through the loop, it wouldn't hurt if they had eased it up a bit when Sonic 1 was made..