The recent discussions about magical items, XP, and the "Leprechaun" class have reminded me that, while I feel I have a good grasp on OSR-style play and design, there is one rule I simply don't understand.
In early versions of D&D (as well as the Leprechaun class being suggested), the character's ability scores do not give advantages, bonuses, or any other clear mechanical advantage. However, for the "prime requisite" of the class, if your stat is high enough, you earn a surplus (typically +10%) on XP gain. This means that an adventure netting 1000 XPs gives you 1100 XPs if your Strength (for instance, for a Fighter/Fighting-man) is 16 or higher, or something along those lines.
What's the purpose of this rule? I don't see what it encourages, supports, or helps formulate.
The only thing I can potentially see is that it acts as a "carrot" - a benefit offered to players who roll high stats in certain areas - to choose a class appropriate to your stats. For instance, a high Strength character should choose to be a Fighter.
What's your take? This one mystifies me quite a bit; I'd always be tempted to cut it out, as later versions of D&D did.