Virtually everything about piano playing is slightly less obvious than you would at first think. You just press down the appropriate keys, right? Almost certainly you already know that there's more to it than that.
At the most basic level, which fingers to set where is a question that opens a whole world of possibilities. A good teacher knows this world and can tell you which fingering will work well for your hand size, your level of expertise, your strength and your memory.
Hitting the right notes the right way becomes incomparably easier if you learn the right wrist and arm movements to guide the fingers. A lot of these are, in fact, rather counter-intuitive. A teacher knows these movements and can observe how well your movements work, and what you should change. (It's virtually impossible to observe yourself as well as someone else practiced in the art.)
The same goes for interpretation. The point of music is not to reproduce notes accurately - for that, put on a record, or a MIDI renderer. The point is to capture whatever it is that a piece expresses, and to participate in the joy or calm or pain or anticipation that it is about. A really good teacher can advise you shape your choice of tempo, articulation, dynamics etc. so that it creates the desired overall impression. Most importantly, through practice a teacher can enable you to make those choices for yourself, opening up the entire boundless world of music out there.
Those are just some aspects - there are many others, but the most important principle is that feedback by am empathic master is an incomparably more efficient way of learning than self-study and trial-and-error.