That's a good question -- I mean, they will run through the code and make sure it compiles or runs as specified. They will find and provide you with technical recruiters from the community (with promises of their name appearing in the list of technical reviewers). But Packt themselves do not add anything as far as I can tell besides printing and publicizing.

Distribution! They have an audience, and they know if they publish x books per week, they will make y $$$'s.

But that is a fair question for an author to ask. If you already have an audience, then you would seriously look at what value they bring.