The PDF document handler is built upon a large corpus of code within MuPDF that deals specifically with the objects, structures and operators found within a document. This code is collectively known as the PDF interpreter.
While it is perfectly possible to use MuPDF to open documents and render pages from a PDF file without understanding anything at all about the PDF interpreter, there are many situations where deeper access to the interpreter can be advantageous.
For example, in order for PDF documents to have their pages rearranged, or files embedded/extracted from them, simple access to the underlying PDF document structure is required.
In fact, the access given to a PDF document structure is such that almost any operation can be coded for.