Pianos normally have 88 keys, ranging from A0 to C8. One of Bösendorfer's pianos go down to F below the low A. The other model goes down to C. How did this happen?
These extended Bösendorfer ranges go back to Busoni's day. He wanted to match the range of pipe organs, as he was making transcriptions of J. S. Bach's organ works at the time.
replete's answer is correct that the original reason was to have a bigger range, as needed for some organ music. However, I don't think that's the reason those Imperial models are so sought-for over all these years – actually playing the lowest notest is scarcely musically useful.
The reason why people want Bösendorfer Imperial is that they sound awesome, even when the low strings aren't played. This probably has two main reasons:
- The big sound frame is better at transmitting all frequencies, in particular those on the low end of the regular range. These strings aren't already at the very limit of what the resonator can do, like they are on many other pianos, but still in the range of what it can comfortably do, so thundering octaves actually thunder and don't just “clank”.
- The low strings give a denser sympathetic-resonance spectrum, when the pedal is pushed. As a result, a Bösendorfer has a richer sound even in high, soft, legato passages.