Almost every iron set has a standard shape for the irons from 3-7 but the 8 and up is different.  Higher more rounded toe – like a wedge.

Why?  Do they do it because everyone does it or is there an actual design principle behind that for the types of shots played?

I suspect that its deeply rooted in the history of the game, formerly before Wedge/Sandwedge and Iron numbers was invented,  the equivalent of 8/9 iron was called a Pitching Niblick,  and thus designed for pitch shots, so designers have continued with that concept ever since. Hence my bugbear – i dont want a <40 degree pitching clubs in my golf bag….  :)