I just took over care of a pool with the house we bought. The prior owner told me the pump doesn't have enough suction to run both the bottom drain and skimmer at the same time (there's a valve position for both... I think). She left it on skimmer unless leaving the house for a while or during a storm, in which case the bottom drain was selected to not worry about clogging the skimmer basket.
On day 1 of having the house, I tried it on "both" and the next day the pool was a cloudy, algae mess. I think it was just bad timing because the chlorine was zero. I shocked it back to life and now it's nice and clear. I'm just confused on what I should be pulling from for the filter.