No instrument possesses the wide-ranging stylistic potential inherent to the piano. Many instruments can be comfortably played in a multitude of styles, but only piano can be consistently found in nearly every musical genre.
It is within the classical medium that piano is often considered the most versatile instrument, occupying the largest concert halls and the smallest salons. Recitalists make use of pianists as collaborative artists. Major symphony orchestras make use of pianists in orchestral works, like the prominent piano solos in Stravinsky's Petrushka. Chamber ensembles make use of pianists in trios, quartets, and quintets, perhaps most famously the Piano Quintet in A by Schubert, the “Trout” quintet, as it's known. These are in addition to the overwhelmingly large piano solo repertoire. The unmatched flexibility of the piano allows for its use in every role, solo and collaborative.
Pre-Modern Classical Solo
Even before Bartolomeo Cristofori's pianoforte, keyboard instruments formed the backbone of musical composition and performance. The organ, clavichord, and harpsichord are the most recognizable to our ears, but the family of keyboard instruments also includes the hurdy-gurdy, cembalo, virginal, and spinet. In its current form, the piano is frequently used to play the works of Bach, Handel, Telemann, and Scarlatti, even though most of their works were originally written with organ or harpsichord in mind.
More than 300 years of classical music is available to a pianist of even modest ability, and in many forms. From the aforementioned composers, to Haydn, Mozart, Beethoven, Brahms...the Liszt goes on.
Modern/Contemporary Classical Solo
By the beginning of the 20th century, the piano had long eclipsed its forebears as the dominant keyboard instrument. The time for experimentation was ripe, and many composers were keen to use piano in previously inconceivable ways. Impressionists Maurice Ravel and Claude Debussy wrote extensively for piano, though not as exclusively as the under-appreciated Erik Satie, who focused almost exclusively on piano.
These have joined a compendium of composers with whom aspiring pianists must be familiar. Less familiar will be works of futurists, modernists, and expressionists, whose demanding compositions require more skill (or in some cases, more industrial machinery) than the average pianist possesses today.
John Cage famously wrote a series of works for prepared piano, which involve attaching nuts, bolts, and screws to the strings, something that should only be done by a professional to avoid serious damage.
The collaborative pianist is the most flexible pianist of all. Sometimes mistakenly given the woefully inadequate moniker accompanist, collaborative pianists are the musicians who make recitals, as well as chamber and orchestral concerts, possible. Soloists might consider themselves experts on a single composer or style, and thus focus on the concertos, sonatas, and other works in their chosen area of expertise.
Collaborative pianists, however, are required to perform vastly different styles, even on the same concert program, deftly moving between contrasting styles like Beethoven and Hindemith. They work with singers, instrumentalists, small chamber groups, and full orchestras. They are often asked to transpose, sight-read, or even read figured bass, skills that are not necessarily prized by soloists. Non-piano instrumental sonatas are considered solo works by the non-pianist, but the collaborative pianist knows better. It is a duet, where the piano part is frequently more difficult than the so-called solo part. Should you find a collaborative pianist that doesn't cringe when they hear the name Hindemith, pay them whatever they ask.
The piano is a truly marvelous instrument, with unmatched capabilities spanning all musical genres. What's more, a young pianist need not determine their intent until after many years of playing. The possibilities are unlimited, and many pianists excel in both solo and collaborative roles. Few instruments possess the ability to fluidly transition between so many styles, and this is why piano has become the most studied instrument worldwide, by a wide margin.