"We have entered into a variety of relationships with the publishers to provide access to their fee-based research through the SSRN eLibrary. These papers are identified by this icon: Incl. Fee Electronic Paper (a red dog-eared page with a "$"). If you are purchasing a fee-based paper, you will click the 'Add to Cart' link and be linked to your Shopping Cart."
So they run as affiliates for paid-for papers.
They also run a job listings service.
You noted already the ability to get a printed and bound copy of a paper on demand.
Edit: without looking in detail it seems those running the service are probably supported by academic institutions [for other work that they do?].
"We have entered into a variety of relationships with the publishers to provide access to their fee-based research through the SSRN eLibrary. These papers are identified by this icon: Incl. Fee Electronic Paper (a red dog-eared page with a "$"). If you are purchasing a fee-based paper, you will click the 'Add to Cart' link and be linked to your Shopping Cart."
So they run as affiliates for paid-for papers.
They also run a job listings service.
You noted already the ability to get a printed and bound copy of a paper on demand.
Edit: without looking in detail it seems those running the service are probably supported by academic institutions [for other work that they do?].