you can enable aftertouch for sequencing on page 2 of record and edit, but the default is off because it eats up events - decreasing the number of notes you can record, but for single songs I doubt you would notice. For 10 songs in the sequencer at once you might.

any control can be assigned to aftertouch to any voice in sound edit, in fact you can assign 2 independent controllers with graduated effect. Obvious uses are overblow on wind instruments etc but even on guitar you can put modulation on the string decay...