The built-in VCL contains procedures that are included and run last by Varnish.
They can complement user defined VCL by providing logic that is appropriate for the majority of the sites. For example, skips cache for POST requests, and/or in presence of cookie or authorization headers.
If some of the built-in logic is not needed, a user can add
return() call from a procedure where built-in VCL logic is not desirable.