As a musician, one of the issues I've had along my experience is the question of what to play. Also, it's a fact that nowadays almost every guitar player starts by reading tabs, so the repertory just sticks to other musician's covers.
It is here where this question enters: "How to make original music?". Here is where musical theory comes in, as a quality musical product often requires to know scales and modes, other than just a good ear and creativity. Improvisation is the key to many of the greatest melodies know to man and is one of the best tools for composing original music. Furthermore, it allows the musician to express without the need of written notes on a score and to roam free around all the possibilities a musical scenario offers.
This is why improvising is so important to become a full-developed musician and to compose original music. It is what you come up in the moment what matters, not what you have come up time ago and just do a cover of it now.