If you cast Knockdown on a creature that hasn't yet acted this round, then yes, at the beginning of its action phase it "gets up" (is no longer incapacitated) and can take its actions.
You could use Knockdown as a way of removing a guard marker on a creature, or to (temporarily) knock a flying creature out of the sky, or to keep a creature from hindering your movement. But Knockdown can be especially effective when targeting a creature that has already acted, because that creature will incapacitated and vulnerable until the end of the round.