The piano was invented by Bartolomeo Cristofori, an Italian master harpsichord builder, in about 1700. The forerunner of the piano was the clavichord, an instrument that was capable of dynamic shading but wasn't sufficiently loud enough for concert use. The harpsichord was the instrument of choice in concert, and it could be made to play loud enough, but the variety of dynamics was also limited. Enter Cristofori's pianoforte, (soft and loud), but this too was too delicate in tone for concert use. It took many improvements in the original before the birth of the massive concert grand piano we all know today.
The piano of Beethoven's time was closer to the original of Cristofori's than the modern piano. There were different makers and each one had their strengths and weaknesses, but they were all similar in that except for the strings and tuning pins, they were entirely made of wood. The wooden frames of Beethoven's pianos could not withstand the string tension of a modern grand, thus they did not have the sonority, volume, or the durability. A strong player like Beethoven was forever breaking strings and hammers. That's not to say the instruments weren't expressive. Modern reproductions have shown how beautiful they could sound, but their voices were smaller. They could not be heard over a full orchestra, hence composition techniques resorted to a kind of 'call and answer' technique where the orchestra would state the main themes, then the piano would enter either solo or with a low volume accompaniment from the orchestra.
This compositional technique can be heard in Beethoven's Piano Concerto No. 5 at the very beginning. The first movement begins with a loud chord from the orchestra that is answered by a piano cadenza. This happens three times before the exposition of the movement actually begins, and parts of the cadenzas are heard throughout the movement. Beethoven had already placed a cadenza for the solo piano at the beginning of the 4th Piano Concerto , but in the 5th piano concerto the cadenzas are of a more dramatic nature. After the orchestral exposition the soloist has his say about them. The themes are explored further in the development section and when the traditional place for a cadenza appears during the recapitulation, Beethoven makes it clear that there is to be no extemporizing by the soloist by writing as much in the score. The entire first movement is dramatic and Beethoven at his most majestic. Beethoven also has the piano and orchestra play at the same time more frequently. The entire concerto is almost written for a piano that didn't exist in Beethoven's time, for the coming of the iron-framed piano and resultant higher string tension and brilliance (not to mention volume) was years in the future.
The second movement is a beautiful Adagio, in direct contrast to the heroic first movement. The second movement segues right into a rondo finale that is full of energy. The theme of the rondo is heard repeatedly and developed along the way until a short duet between piano and timpani leads to the ending flourishes of piano and orchestra.
Beethoven's music in general and this concerto in particular is a good example of how a composer's talent, insight, ingenuity and creativity can change their art in many ways. The piano was never the same after Beethoven. It couldn't be. Beethoven demanded so much from his instruments and players that they both had to evolve and learn new ways and methods to express the music that he wrote.