Many ZDaemon servers use custom PWADs (user-created levels, graphics, sounds etc.) and some people do not have fast access to a website to upload these WADs. WADServ allows users with fast connections (cable, DSL or better) to either host the WADs used by the server or set up a simple website.
WADServ is relatively easy to setup (compared to Apache/OmniHTTPD et al.) so that novice users can set it up quickly and easily. Once set up, GetWAD can find and download WAD files.
WADServ is built into ZSL, although it is disabled by default.