Pre take off check arm hatch, disarm during after land checks. The hatch switch box has two similar, side by side switches, one of which is guarded, the unguarded switch selects hatch arm and the guarded switch blows the hatch. With the hatch armed, when the seat ejection sequence is initiated the hatch explosive bolts fire and the hatch hopefully will disappear, once it has, the seat can travel up the rails and out into atmosphere. The hatch is also able to be blown without ejection being initiated for the reasons stated by others here, ie; wheels up landing or action requiring escape via the back of the cabin.
Low speed hatch blowing is not recommended as the hatch needs enough forward speed to carry it away, if stationary the hatch will lift a small amount and then very likely fall back into the hole it just left, it's very heavy with sharp edges!!!