Not sure really. I'd imagine there have almost always been fringe groups of this or that ideological variation, but dirtbag left does seem to be somewhat recent. Or perhaps the flocking of some leftist academics to such a place, that could be new. Justin Murphy comes to mind, think he was the one I first heard use the term. He also used heterodox left to describe himself, which is a term I often use as well.
I think there is just an awkward transition involved in realizing that capitalism can be liberal, not to be too reductive.
One way an alt-left can serve the greater political landscape is by functioning as a liaison between the institutional left and the alt-right.