People had been playing electric guitar for decades before Jimi Hendrix. Mostly it had been used as a louder, less effortful version of the acoustic guitar. Jimi was one of the first to think of the guitar amp as a musical instrument unto itself, an early analog synth, with the guitar as a very sophisticated control surface.
Say “oooh” as in “noodle.” Then say “aaah” as in “park.” When you say “oooh” your mouth is more closed, with less resonating space and a smaller opening. This configuration blocks the higher overtones of your voice. When you say “aaah” your jaw and lips open, creating more resonating space and letting more high overtones through. Now glide from one to the other. The resulting “ooohaaaah” is the sound the wah-wah pedal is named for. By selectively filtering an electronic instrument’s overtones, the pedal can make it sound more vocal. It’s only two vowel sounds out of the dozens your mouth is capable of producing, but it’s a start toward making a more human tone.