Webcal was initiated for use with the Apple iCal application and has become a common de facto standard for accessing iCalendar formatted files via WebDAV. It is not an official URI scheme, such as http and ftp, as registered with IANA. The Webcal protocol prefix is used to trigger an external protocol handler which is passed the URL of the .ics file rather than being passed the downloaded contents of the file, in much the same way feed is sometimes used to trigger external RSSreaders. The idea is that with this protocol prefix the target file should be subscribed to rather than imported into the calendar application as would happen with a simple download.
CalDAV and GroupDAV are both efforts to provide WebDAV-based access to calendar stores with finer granularity. However, neither of those protocols call for using dav style URIs. Instead, both drafts call for using the HTTP OPTIONS feature to return that the server supports calendaring extensions.