ToME: the Tales of Maj'Eyal

Is there a purpose to them I’m not seeing? After level 3 when you can wear plate armors nothing else is listed for levels 4 and 5 and none of the numbers except the effective level change. I was assuming that something would unlock for skilling it all the way up (especially since it has that line at the top about it affectin the world) but I skilled it all the way up and nothing happened.

The numbers won't actually change if you're not equipped with heavy or massive armor, I'm guessing that was your problem.
Other than that, when equipped with either of those, you'll see the values go up with each level, massive has more than heavy.