[MENTION=23751]Maxperson[/MENTION] back in the 90s Elminster was for FR what Mordenkainen and the rest of the Circle of 8 was for Greyhawk, and Fizban for Dragonlance.
FR back then was an interesting alternative setting with its own flavor, you had the undermountain / underdark myth drannnor, things like dead magic and wild magic zones and mythals. These were very much FR gimmicks, like Psionics in Athas or dragonmarks in Eberron. It was new and cool and the setting was not such a kitchen sink as it is today.
NPCs like Elminster back then served some purposes, being a mentor, being the balancing force of good versus many evil threats and therefore logic:
Imagine FR with all the drow and orc hordes Zhentarim, undead, some eveil dragons: Who keeps them at bay if they go for an all out attack? There have to be some strong forces of good to keep up a "balance of power" for pure logic reasons. NPCs like that could interfere in scenarios to even the odds like a when an end boss would summon an avatar of an evil deity or some demon overlord to his aid in the end fight, a sort of Gandalf on the bridge scenario, the players still got to fight the end boss but the out of their league mob is occupied with Elminster.