you can assign them to control certain parameters in the software,
for eg. you can assign a knob to control the cut off fq' of a synth,
and a fader to control it's volume.
you assign them to almost any parameter in the sequencer or virtual instrument,
giving you "phisical" control over software features.
you can of course control all these features with a mouse
and use the mikledet only for recording notes.
it's a matter of comfort and ease of use.
advantages are better, more acurate realtime control.
different mikladot will have different controlers and options
(knobs, faders, mod wheels etc.).

adam