[RPG] What are ability scores (not modifiers) used for

It is better to take the array.

There are several reasons for having all players start with the same points then choose their stats:

1. Consistency - You'll never have to worry about a player who rolls poorly being stuck with bad scores across the board.

2. Fairness - You won't end up with any players rolling really well and having multiple high scores. Characters with those ability scores can often overshadow others.

3. No Cheating - You won't have to worry about players fudging their dice rolls.

4. No Upset Feelings - You won't have to deal with any player jealousy's over another player having better ability scores.

The History

This actually goes all the way back to the first OD&D supplement, Greyhawk. The maximum spell level a Magic-User could cast was now limited by his Intelligence. Although interestingly, Clerics were explicitly not limited by Wisdom. The justification was that, unlike Magic-Users, a Cleric's spells were divine gifts, not based upon their skill. Intelligence also limited how many spells the Magic-User had.

Speculating, this was probably partly simulationist and partly for mechanical reasons. Arcane magical ability is tied intelligence, both lore-wise and in the the Magic-User's prime requisite was Intelligence. High level spells are more complex (the spell level system is a direct mapping from Chainmail's complexity system) so there's logic to a smart Magic-User being able to handle more complex spells than an average one.

Your Ability Scores had little mechanical impact in OD&D, primarily a bonus to experience gain. Greyhawk began increasing the existing, minor bonuses and penalties and adding new ones. Intelligence affecting Magic-Users spells was part of this increasing affects from Ability Scores. And only Magic-Users with an Intelligence below 11 were actually losing anything compared to the core game, because the 7-9 level spells were added with Greyhawk, and did not exist before ability limits.

This Ability Score limit spread to Clerics with AD&D. High level spells required a certain Wisdom and a low Wisdom could cause spell failure. You may have noticed that Ability Scores were only limiting the casting of high level spells. As KRyan touched on, this is because of Prime Requisites / Ability Score requirements. RAW, you couldn't even play a caster with a below average Ability Score in their Prime Requisite.

The Implications

So there's the historical precedent, which explains where it comes from. Was there any other reason to carry it forward, beyond tradition? It makes your Ability Score have a greater effect on your casting. The fact that spell strength is often not based on your Ability Score seems like it could be unbalancing to ignore the Ability Score requirement. I doubt it would be that bad, but I'd ask a optimization expert about it.

I personally don't have a problem with the idea that magic is too complicated for the average (Ability Score 10) person to grasp, and the smarter/wiser you are the more complicated spells you can comprehend and harness. It seems intuitive me, but if it doesn't to you and your players, I say house-rule away.