Think of music theory as the "5th quarter" of your balanced practice session, after Scales, Chords and Arpeggios, Reading and Repertoire. Theory knowledge enhances all of your playing activities. Just as an example, assuming you know your basic chords, there's no reason you shouldn't be able to strum the chords to "Happy Birthday" in different keys to suit different voices, higher for children, lower for adults. And it's common to get music for a song you like printed in a less-than-ideal key, and you ought to be able to change and improve it. When playing in a band, you'll have an easier time keeping track of where the song is if you're thinking of the chord functions and large form as you play. Just to do better with tuning, you need to know a bit of theory.
