In Chronicles: Volume One, Bob Dylan wrote that he couldn’t remember when it occurred to him to write his own songs, but he did explain why he began:
Opportunities may come along for you to convert something – something that exists into something that didn’t yet.
The language is surprising, and telling. Conceptual artists don’t innovate by making things that are entirely new, but by recombining and recycling things that are old, creating unexpected syntheses of earlier styles or existing works within their discipline. Dylan understood this, and he wanted to follow the granddaddy of modern conceptual innovators:
Picasso had fractured the art world and cracked it wide open. He was revolutionary. I wanted to be like that.
Picasso had created an unlikely synthesis of Cezanne, Gauguin, and Egyptian and African art to make Cubism; now Dylan brought Rimbaud, Brecht, and Allen Ginsberg to folk music:
What I did to break away was to take simple folk changes and put new imagery and attitude to them, use catchphrases and metaphor combined with a new set of ordinances that evolved into something different that had not been heard before.
Dylan didn’t want to compete with professional songwriters:
Nothing would have convinced me that I was actually a songwriter and I wasn’t, not in the conventional songwriter sense of the word. Definitely not like the workhorses over in the Brill Building, the song chemistry factory…Over there, they cranked out the home-run hits for radio playlists…all the popular songs, all the songs with crafty melodies and simple lyrics that came off as works of power over the airwaves.
What he brought to popular songwriting was intellectual:
One thing for sure, if I wanted to compose folk songs I would need some kind of new template, some philosophical identity that wouldn’t burn out.
He believed he achieved that – “see into things, the truth of things – not metaphorically, either – but really see, like seeing into metal and making it melt, see it for what it was and reveal it for what it was with hard words and vicious insight.” Years later, after his gift had abandoned him, he could look back with pride:
I’ve written some songs that I look at, and they just give me a sense of awe. Stuff like “It’s Alright, Ma” , just the alliteration in that blows me away. And I can also look back and know where I was tricky and where I was really saying something that just happened to have a spark of poetry in it.
Editor's addition, link: “It’s Alright, Ma” 
Related articles by the author: