To my ears the wah pedal has always added a very vocal sound to lead guitar lines. A great example of this is Joe Satriani's beautiful lead guitar in "Summer Song". I try to demonstrate the vocal quality of the wah in this guitar lick. Notice how it changes the way I phrase the guitar lick. This wasn't conscious I tried to play the lick the same but I find playing with the wah pedal actually makes my phrasing try for a more vocal sound. Sounds weird but hopefully that will make sense when you see the video. In this guitar lick I also used the whammy bar which is not reflected in the notation.