WebScarab is basically a framework for running plugins. It provides a foundation for the plugins to generate requests and responses (aka conversations), keep an audit record of these conversations, and review what those conversations were, as well as a tree view of the URL's that have been "seen".
It also passes all of the conversations that the framework sees to all of the plugins that are installed. This allows each plugin to perform some analysis of each conversation.
Other services that the framework itself provides are handling of upstream proxy servers, client side certificates, a pool of cookies that have been seen or set by various plugins, and a convenient way to perform various encodings and decoding of strings.