Midi controller question

[nutrinoland]

Hi..i have a midi controller..evolution mk 425 c
it has assignable knobs and assignable buttons.
I am not able to figure out what the assignable buttons do...i use fruity loops...when i link a button to a certain parameter,all it does is to set the parameter to zero when i press the assigned button...thats all...nothing else...
how does it work....what are assignable buttons for on midi controllers??
if you can, pls give me an example of how to use it in fruity on a certain parameter...

thanks..