Wah has several practical applications:
- try playing a natural harmonic or two with a clean tone, say at the 12th fret, then rock the wah forward each time you want a new 'attack' - works well in conjunction with a delay for a 'jewel-like' sound.
- when soloing, press the wah into full treble when you reach a high sustained note in a phrase having started from heel position
- press wah forward to accentuate individual notes in a phrase, for example, if I play a, b, c notes on high e string at 5th 7th and 8th frets, pressing wah forward on each note gives a 'speaking' quality
- play muted chord 'stabs' and wah in time with rhythm to create the wakka-wakka noise
- play ascending runs from low positions with the wah in heel and as you go up gradually move the wah into full treble - will give the run a sensation of coming from the depths
- play the opposite or vice versa, i.e. either start high and go low moving from high treble position to heel, or start high and go low moving from heel to toe position, etc.
...the only boundary is your creativity. To me the wah is indispensible both for leads, rhythms and making weird soundscapes.
btw, wahs work best with your bridge pickup - more sweep is available.