isn't that what touch sensitivity is for?
Touch sensitivity is use for the volume and charasteristics of a single note.
With a volume pedal you can controll the volume of all notes of a single performance... you can create certain effects this way, that you could never do using touch sensitivity.
All great organ performances from the past where played on non touchsensitive keyboards and to recreate those performances, you require the volume pedal for the volume controll.
Or when playing a sax, you can slowly increasy the loudless of the note you're playing with the pedal (as a real sax player could by blowing harder) while your touch sensitivity only allows you to controll the initial volume of a note.
In such way to recreate accoustic instruments, a volume pedal is an awesome tooll too.