> the brine pond.

Then why not store the oil there directly instead of going all the trouble of pumping it in and out of the caverns? Is it so it doesn't rain on it?

Crude oil is sketchy stuff. All sorts of VOCs, etc evaporate from it. Some valuable, some, flammable, some will merely kill you. Keeping it confined makes all that more manageable.

TFA lays it all out, oil out in the open is exposed to attack/lightning strikes/etc.