Well, certainly unexpected, but some things are just good and become a classic. And yes, they do go for thousands of dollars. In the case of the Hammond, it se

Well, certainly unexpected, but some things are just good and become a classic. And yes, they do go for thousands of dollars. In the case of the Hammond, it seems to be mostly by accident. They were just trying to create a cheaper version of a pipe organ, in effect. The tone wheels made it possible. The drawbars allow changing the timbre in a lot of ways, which makes the instrument expressive and configurable. You can adjust the timbre while playing a note! Sort of like using a mute on a trumpet, only more combinations. The keys make a satisfying percussive click when played, which is also expressive. A Hammond player told me once this click was not intentional (the designers viewed it as a bug), but it helps give the instrument a nice attack, which helps make the starting of the notes apparent to the listener and allows the instrument to be used rhythmically. The Leslie rotating speaker was an attempt to create some of the acoustic ambiance of pipe organs. (Which is amazing, by the way. The instrument literally surrounds you in some cases. It's like centuries-old surround sound. Not to mention the rich acoustics of a church hall, which is built to encourage reverberation so that the audience could hear an orator in the days before amplification.) But the Leslie speaker's rotation adds not only spatial effects but also doppler. And the speed of rotation can be changed, so you can make the sound tenser or more mellow by varying the speed. So it's great for expression as well. Incidentally, a lot of other instruments achieved greatness by accident. Resonator guitars (Dobro, National Guitar, etc.) were made in an attempt to get more volume, but they ended up having a unique sound. Electric guitars were made for more volume too, but they opened up an entire new world of tones, as well as different playing styles due to lighter strings. The stereotypical distortion effect on electric guitars (that give them their aggressive "bite") came about because guitar amplifiers just didn't have enough power. The vocoder was developed for speech synthesis and telecom, but it found use in music as a way to give a radically different timbre to a human voice. (Or is it to modulate another instrument's sound to follow the volume of a human voice? Maybe those mean the same.)