I had been looking for a good wah pedal for a while, and when I showed my friend one I was thinking of getting, he asked why didn't I get a multi-effect pedal. I found one for the same price as the pedal I was looking at, the Digitech RP50, which had wah on it, just what I was looking for, I thought.. It seemed like a deal, but then today when I got it I find out to use the 3 settings of wah, I have to connect a passive volume pedal to it.

My questions are..
So I really have to have a passive volume pedal to get the wah?
What does a passive volume pedal do?
and why do I need one in the first place, why didn't it come with adjustable wah? (not including the envelope filter)

The envelope filter wah sounds okay, but it messes up the sound when I play, and I don't really like it.. I'm starting to think I should've just paid $10 more and got a Dunlop CryBaby.. But could someone explain this whole passive volume pedal thing to me?