It all boils down to the tone sounds. I just bought a book on Orchestration which is really insightful and helpful. Thing about it like this: On which instrument would an incredibly fast and soft run sound better on? Piano or Harp? Which instrument could be better used for forceful, strong, powerful sections? Brass and Timpani, or Piano? Which would be better for a soft, melodic, part? Flutes and other woodwinds, or Piano? Which has a stronger brilliance and an almost human voice? Violins or Piano? Which can best convey emotions based solely on the sounds when put together, Orchestra or Piano? But, which instrument has small indiosyncracies that make it much more useful for some parts, like a pedal, or the largest range of pitches? Piano or Orchestra? And which instrument can, say, rubato, be best played on? Piano or Orchestra? Think about these for a while, and it all makes sense. Does this mean the orchestra is superior to the piano? Not necessarily. I can't imagine the Rachmaninoff Piano Concertos being played on any other instrument
! It is in a way a supplement to the orchestra. The piano is much more easier to write for, or at least faster, usually. Orchestral pieces take amazing amounts of time to compose. The piano also has certain qualities which make it better for some areas better than any orchestral combination. Also, a funny thing to consider is that with a rather expensive grand piano, you could probably buy one of the necessary orchestral instruments instead, and end it might end up costing you less!
Answer to question #2: No. Can you imagine the Liszt Bminor sonata for orchestra?
I could, actually, very easily in fact. I have all the instruments and sounds sorted in my head, but would never be able to write them down. In fact, it sounds spectacular as an orchestra! If only others could hear what I hear. It would start off like Liszt's Les Preludes. The notes are staccatissimo, so I would use Cello Pizzicato. Then we would use arco for the next few notes. Repeat process. Then the final two pizz. notes, followed by a large tutti of the orchestra for the quick crescendo. After a part of that, with a short rest and then violins play that softer part. Then we have a long rest, which is followed by a small bass rythm using cellos. The melody would be brass. Then come the fast arpeggios downwards. They would be woodwind. Then comes back the loud motif, which would have basically a tutti, followed by the bass rythm which leads to the next new part, fast arpeggios(strings and woodwind) and a leading motif(brass), which comes to the right hand playing a faint melody and the left hand on the higher registers playing a simple but demonic melody, would be replaced by: Left hand melody with brass, the right hand would be woodwinds and strings. I could keep on going! Kind of fun.
Topic: Is it possible to orchestrate any piano piece?