Music theory makes music beautiful in the same way that understanding that snowflakes are made of delicate unique structures makes snow beautiful.
Music theory isn't about how well you can read sheet music, but it makes reading music easier. It doesn't tell you what to do, but it lets you understand and communicate with other musicians. It also lets you sometimes see why certain musicians sound the way they do. Giving you a method for impersonating their sound and working it into your natural musical vocabulary.
Sometimes music theory gives you a great method for composing songs. Or a clever way of creating a musical puzzle for the heartiest of musicians.