I can't really give any logical answer because 1000 gaming groups might have 1000 different reasons to do stuff. But OD&D might have encouraged folks to seek out all the rumors and kill every last monster in them:
OD&D: As characters meet monsters in mortal combat and defeat them, and when they obtain various forms of treasure (money, gems, jewelry, magical items, etc.), they gain "experience". This adds to their experience point total, gradually moving them upwards through the levels.
I'm not really going for the sword. The DM said there's monsters. The only way I go up in levels is to kill monsters & get loot. And, I'm expecting +1 weapons at 1st level cause the DM has lots of nasty monsters immune to non-magical weapons in their Monster Manual. It's not like 5E and its fancy "bounded accuracy" math which may not hand one out till 4th level or later.