The dev wants not only to test that a particular HTTP request sent to Tiki produces the expected result. He also wants to verify that it's actually possible for an end user to issue that HTTP request from another given page.
For example, if you are testing page editing, you want to check that tiki-editpage.php displays what it should when it is invoked with appropriate CGI arguments.
But you also want to verify that it can indeed be invoked with the proper arguments, from a given tiki-index.php page with given arguments.
Let's take a simple (albeit unlikely case). Suppose I do a change to the code which ends up removing the Edit which, previously, was visible at the bottom of a given tiki-index.php page. When that happens, the test have to be able to spot that.