Also worth noting that you can get multi-release cleats for SPDs, which allow you to roll your foot out as well as pulling your heel out sideways. They're quite useful if you're new to clipless and want to be able to get your foot out in a panic. I've never moved away from them TBH.
Ok thanks. What is the difference then? I mean between SPD and SPD-sl? Why did Shimano have to devise new stuff for the road?