Mutool is a collection of useful command line utilities rolled into a single executable.
As explained earlier MuPDF is a C library that encapsulates all the smarts required to open/render/manipulate document files of a range of a formats. This means that most utilities using it are reduced to very thin shells that just call down to the library.
When you consider the additional factor of the size of the resources built into MuPDF (fonts, CMAPs etc), this means it makes a lot of sense to build a single executable with multiple utilities sharing a single copy of the library.
If you run mutool at the command line with no arguments, a list of the possible options will be displayed: