As other have mentioned, there are a lot of things for which we can use pedals.
I have FC-7 pedal, however I don't use it really. I think the typical use would be for playing organ... listen to
this song and pay attention on how organ volume changes -but believe me, it's much harder to accomplish this than one might think. FC-7 really needs a lot of practicing.
I mainly use normal sustain/switch pedals (sustain pedal is switch actually, it only has different shape and feel). I use pedals for changing something that's hard to do while both hands are busy playing: switching from one style variation to next one, repeating variation (to insert fill-in), to switch trough registration sequence, to change OTS voice, etc. In short: that way I have my hands free for playing only, without the need to push buttons while I play.
Yes, sometimes I wish SX700 would have 3rd jack for pedal
Bogdan