Adventure cards are, for most practical applications, modal spells. You can play them as a creature and then they behave normally as a creature would, or you can play them as a spell and exile them upon resolution. If you do the latter, you can cast the creature from exile at any point. Basically, you can choose to cast the creature, or you can choose to cast the Adventure AND the creature, but you have to cast the Adventure first. With some cards, such as Lovestruck Beast, the sequencing is obvious. You're supposed to cast the adventure and then the creature. In other cases, such as Flaxen Intruder, you're supposed to cast the creature and the Adventure is just a kicker of sorts if you draw it later in the game.
The preview card Wizards of the Coast gave me is really one where the sequencing is not clear and that could go either way depending on what the game looks like, which is just how I like them. World, meet Hypnotic Sprite in both normal and promo version!
Neither side of Hypnotic Sprite is particularly great on its own, though I suspect that's going to be the same for most Adventure cards. Here, what you're getting is flexibility.