In theoretical computer science:
Papers are written, illustrated, and typeset (in LaTeX) by authors and refereed by unpaid volunteers. Some publishers give free journal subscriptions to editorial board members; a few pay a small stipend to the editors-in-chief of each journal; otherwise, editors are also unpaid volunteers.
Most journal publishers provide an online system to help editors track submissions and communicate with referees. Conference publishers do not; most program committees use free systems like EasyChair or HotCRP.
Some journal publishers employ copy editors, who produce the final camera-ready paper directly from author-provided LaTeX and image files. Specifically, copy editors correct (and invariably insert) spelling and grammar errors, and reformat the paper (especially the bibliography) to fit the publisher's standards. For other journals and most conference proceedings (Springer's LNCS series being a notable exception), copy editors simply do not exist; camera-ready papers are produced by authors using publisher-provided LaTeX packages, except possibly for page numbers.
Most publishers provide electronic versions of their papers to subscribers. Some publishers also provide extensive indexing and cross-referencing of their publication catalog. Online-only venues are still relatively rare, so for most venues, publishers print, bind, and ship paper copies.