Is it important to study music theory when playing the piano? All I want to do is play piece that other composer created.
If you really want to play the piece that the composer created it is essential to learn theory. Theory tells you which notes are doing what, and informs how you play them. Especially as you go back in music history, notational practices become less and less exact. The composers expect you to just know certain things without them having to notate it. In Baroque music, they just wrote in figured bass quite often, and expected the player – using the theory of the time – to be able to "realize" the figured bass to achieve the composers intention (look up "Furno" or the "Furno Method" on wikipedia).
So if you really want to realize the creation of a composer, you need to study theory and history to understand what the composer intended. If you don’t want to perform at a concert level, just want to impress friends at parties, and play soothing music for your loved ones then you don’t need to study theory.