Do you need spare actions available to play Throne Room?
Throne Room costs an Action to play; the card you play twice is part of the effect of Throne Room, and neither play of it consumes an Action.

My speculation is that that particular wording was in answer to a complaint encountered during development, something like: "Village gave you two actions, but you just played that Witch two times with Throne Room, so you have no actions left." That's a mistaken understanding of the way Actions work, of course, but everyone was a novice back then.

Other cards that include allowing you to play other Actions as part of the effect include King's Court (from Prosperity), Cultist, and Procession (both from Dark Ages).