What is the purpose of random encounters?

[KorvinStarmast]

My background is with BX and similar-style games, and less with AD&D, although there are similarities. I think that, as originally introduced, the idea of dungeon delving was to get in and out as quickly as possible with as much loot as possible. This is a system where gold is far more valuable as an XP metacurrency than killing creatures.

Random encounters are meant to speed things along and encourage the PCs to strike a balance between full investigation of each chamber and rushing through (plus, if you're doing a good job mapping, you should be able to tell where a number of secret doors/chambers are based on the map); random encounters do not typically result in meaningful treasure, so you're risking resource drain in exchange for, theoretically, very little.

Plus, the morale and reaction rolls in BX ensure that not all encounters end in combat; in fact, the incentive is to minimize combat. Surprise rolls give the PCs the chance to potentially avoid combat altogether, and most monsters are automatically hostile only on a roll of 5 or less on 2d6 (modified by Charisma if the characters attempt to parley). Morale checks are made, and many monsters will attempt to flee or surrender instead of fighting to the death.

Yes, that style of play incentivized more than combat.
The other key feature of that was "Treasure in Lair" - if you weren't in the lair, there wasn't any treasure, beyond the usual claw, tooth, or maybe body part one might be interested in for making potions or trading to an NPC wizard for something ...