Continuing with the OD&D standard arrangement of ability scores, next up is after Strength is Intelligence.
Within the context of OD&D, Intelligence had two primary uses. 1st, it was the prime requisite of Magic-Users. 2nd, intelligence was used to determine the number of languages a character knew at the beginning of play: one additional monster tongue for every point of intelligence above 10. Like the reference to traps for Strength, Intelligence "will also affect referees' decisions as to whether or not certain
action would be taken" (M&M pg. 10). Once again I'm not really sure what that means. It's almost as if the referee is allowed to take control or somehow limit a character's actions based on his/her Intelligence score.
The addition of Greyhawk brought with it a number of additional used for Intelligence concerning the knowledge of Magic-User spells. Without explaining the meaning of these three categories in much detail, Intelligence then gave a % chance know know any given spell, the min/max number spell of a certain spell level a character could know, and the highest level of spells a magic-user could ever learn ("Only magic-users above 13 intelligence are able to employ 7th level spells" - pg 8).
AD&D continued to build upon the basis of Grawhawk and OD&D. Since a minimum 9 Intelligence was required for a Magic-User character, the tables originally printed in Greyhawk were reworked to fit this new assumption. Furthermore, the number of language a character knew at the beginning of play is changed to the number of language a character could ever learn - thus requiring in game time for non-Magic-Users to make use of a high intelligence. I always though this was strange, especially combined with the fact that only an 8 intelligence was required to learn an additional language; regardless, I've never seen anyone take the time to pick one up as detailed in the Dungeon Master's Guide.
B/X removes much of the complexity of Greyhawk and AD&D, instead based the number of spells a Magic-User has in his spell book solely on the character's level. Furthermore, an Intelligence of 18 only allows a character knowledge of 3 additional languages, unlike the 8 of OD&D or 7 in AD&D. Furthermore, B/X has reading and writing restrictions for characters with a low intelligence - the first time non-Magic-User characters are penalized for a below average score. Overall, Intelligence is likely the least useful score in B/X, even to Magic-Users.