Command and control. One frequent thing that black hats do is have all the boxes they root subscribe to e.g. an IRC channel, so that they can receive further commands. The ratware ("software written with nefarious purposes in mind") often comes pre-configured with options for blasting spam, hosting content on HTTP (for SEO or exploitative purposes), doing DOSes, and executing arbitrary commands against the local system.
One can, of course, envision numerous ways to get data in and out without it being an IRC channel, but that is easy to implement, works across a wide variety of target environments, and plays well with the existing ratware ecosystem.