House Rule: Ability Score Increases

Why would I add such a thing to an OSR game where it doesn't mean as much in terms of character optimization?  If nothing else, I add it simply because I like it.  I always liked the idea of being able to get stronger, faster, or smarter as time goes on.  The influence of the game world can cause it, magic can cause it, etc.  See below for the rule.  Note that my classes all go to 20 levels.  More on that later.

"To represent the growth of characters in the game outside of just their class, at 5th, 10th, and 15th levels, a player may add 1 point to any of his character's ability scores.  No score may improve above 18, of course."