Personally, I think that this (Seekers of Perl Wisdom) is as good a place as any to discuss the "why" of some modules. Probably the reason that we don't see more questions of this type is that module authors in the main make a fair attempt at explaining what their modules do in the POD. It wasn't always thus and still isn't universally, particularly for the more specific modules which assume a certain level of domain knowledge. That's understandable.

As for the specific case of App::unbelievable the author has a section entitled "Why?" in the POD and again in the README. It's sparse (and the module is only about a month old, so more may come) but it gives the gist. In essence, if you are familiar with Dancer2 and want to use it to maintain what will be a static site then this module (among others) purports to help you do that.