Why should agar plates be free of visible moisture?

Answer 1

because when you streak out the bacteria onto the agar plate, you don't want droplets of water to smear the bacteria around on the plate.

if you are spreading the bacteria on the plate, you don't want a great deal of moisture on the plate, otherwise the liquid will take a long time to dry/absorb into the plate.

in any case, a plate that has a lot of moisture in it will full of condensation after incubation at 37C.