You never know who's going to play your game. Sure, most of the players will be aware of what to do from the get-go, but others may have no previous experience with the genre. That's why it's important to offer up a tutorial. You don't want to alienate a newcomer right from the get-go!
Today's dev blog for Piczle Cross Adventure goes into detail on the game's interactive tutorial. You can check out the full piece here.