Monster Advancement

[Flame Effigy]

Step 2: Add Class Levels
Once you have determined the creature's role, it's time to add class levels. The first step of this process is to modify the creature's ability scores. Creatures with class levels receive +4, +4, +2, +2, +0, and –2 adjustments to their ability scores, assigned in a manner that enhances their class abilities. Creatures with NPC class levels do not receive adjustments to their ability scores.

Here's something I didn't know about until today.
What exactly is the purpose of the ability score adjustment?

[Bob Bob Bob]

Monster stats start at 10 or 11. As in, 10/10/10/10/10/10 or 11/11/11/11/11/11 or anything in between. Everything else is ability score modifiers/leveling up. The adjustment gives them the Heroic Array 15/14/13/12/10/8 (11/10/11/10/10/10). I wish I could tell you where that was from, but I honestly don't remember. I think Savage Species (3.0 book)?

[Khudzlin]

Monsters get 11/11/11/10/10/10 (racial modifiers are always even, so any even stat comes from a 10, and any odd stat from an 11). I guess the purpose is to make the monster special (enough to get class levels), somewhat like a PC.