When playing music, rhythm is everything. For some players, toe tapping actually helps them internalize the beat, feel the groove, and maintain the tempo. Tapping might be seen as an outward expression of one's inner awareness of the pulse of the music. Foot tapping can help us play well with others!
Foot tapping reflects dance beats. Think about it. Many of the tunes we play are dance tunes - jigs, reels, hornpipes. Tapping is sort of like dancing to your own music. Whether toe tapping or swaying to the music, allowing your body to move in response to the music is a good way to feel the pulse. It's fun!
Plus, it's good exercise. My pedometer gives me credit for steps when I tap my foot!
How 'bout it? Are you a toe tapper?