Plack is one of the most important projects in the Perl world in the last two years. It's a good idea borrowed from Python's WSGI and Ruby's Rack but made Perlish; it's a simple formalizing of a pattern of web application development, where the entry point into the application is a function reference and the exit point is a tuple of header information and a response body.
That's it. That's as simple as it can be, and that simplicity deceives a lot of people who want to learn it.
Test::WWW::Mechanize::PSGI is one of many reasons why it matters.
Given a Plack application, you don't have to deploy to a web server—even locally—to test your application as if it were deployed. TWMP executes your application as if it were running via Plack (because it is!) and allows you to perform Test::WWW::Mechanize tests against it.
Certainly you need to test that you've deployed your application correctly when you do deploy it, but that's a single test.
Plack and TWMP (and Plack::Test) use the well-defined Plack pattern to make something which was previously difficult into something amazingly easy. They're not the first and they won't be the last, but they do demonstrate the value of Plack.