Tbh, the main reason why I leveled up past 100 was just for the glory. I know that sounds cocky, but it just made me feel like I had accomplished something.
The point where levels become empty is 75, or whenever you have every unique item. I had all of the original unique items a while back from when I originally started playing the game not long after it was released, so everything besides the Green Guard armour and cloak was a bit worthless. But then again, I do think that Green Guard armour could have better boosts.