One of the benefits of this model is that if something causes the child process to die (e.g., a badly written CGI script), it won't cause the whole service to fail. In fact, only the client that initiated the request will notice there was a problem.
Many free (and non-free) CGI scripts are badly written, but they still work, which is why no one tries to improve them. Examples of poor CGI programming practices include forgetting to close open files, using uninitialized global variables, ignoring the warnings Perl generates, and forgetting to turn on taint checks (thus creating huge security holes that are happily used by crackers to break into online systems).
Why do these sloppily written scripts work under mod_cgi? The reason lies in the way mod_cgi invokes them: every time a Perl CGI script is run, a new process is forked, and a new Perl interpreter is loaded. This Perl interpreter lives for the span of the request's life, and when the script exits (no matter how), the process and the interpreter exit as well, cleaning up on the way. When a new interpreter is started, it has no history of previous requests. All the variables are created from scratch, and all the files are reopened if needed. Although this detail may seem obvious, it will be of paramount importance when we discuss mod_perl.