Back in the day, playing Advanced Dungeons & Dragons, we never used the Deities and Demigods rulebook as a “Monster Manual of gods” (though this apparently happened in other groups). We got way more use out of the extended ability scores tables in the front of the book.
Usually it wasn’t due to characters having inherent ability scores above 18 (though those amazing magic tomes got used more than once), but more often to reference temporary ability boosts from magic items, like Giant Strength potions. It was so fun opening the DDG to find what your over-18 abilities meant.
STRENGTH is fairly simple, being a logical progression of the table in the Player’s Handbook, with no new special abilities. But, wow, talk about being a juggernaut in melee and a threat to doors and gates everywhere!
INTELLIGENCE, in addition to boosting magic users in their spellcasting, also conveys increasing immunity to illusions above a score of 18.
WISDOM likewise continues to cumulatively grant more divine spells for clerics. And a very high WIS being would have a similar immunity to charm spells and psionic abilities.
DEXTERITY, like STR, is a simple continuation of the PHB. You’d become increasingly quicker in combat, very hard to hit, and boast amazing thieving abilities if of the right character class.
CONSTITUTION above 18 continues to increase your hit points if a fighter class. (Presumably both System Shock and Resurrection Survival were already 100%). In addition, however, exceptional CON will grant a special bonus to Poison saves, and – hoo-yeah! – Regeneration… though being per-turn, this will usually be out-of-combat recovery.
Exceptional CHARISMA scores extend your potential number of henchmen, their loyalty, and greased encounters by boosting the odds of a positive reaction. A new ability, Awe, can overwhelm those who beheld your eminence.
Alas… “Mortal-born persons who somehow acquire godlike charisma will not radiate this aura.”
Some extraplanar entities could be “so loathsome and repellent as to actually have negative charisma.” They’d radiate Horror to those who dared to look at the abomination.
See Also: Deities & Demigods: Cthulhu or not?