The piano is the foundation of western music. It's range covers most of the pitches audible to human ears and is capable of enormous harmonic complexity. Because of this nearly all classical music by composers such as Bach, Beethoven, and Mozart was first worked out on the keyboard before it was notated for other instruments.
These days the influence of the piano and it's modern counterpart, the keyboard, can be heard in the music all around us. The musical keyboard has become the way most artists interface with digital recording and mixing tools. In addition, with it's physical layout the piano is accessible technique-wise to anyone - from very young children to the most advanced concert and jazz pianists. For this reason the piano has always been one of the most popular instruments to take lessons on.