Yep, it also has benefits to the site. AGL is quite generous with his time in terms of accepting static pin requests, but it can become a difficult situation for large website operators. It's a little nerve-wracking to know that the fastest you can make a change is 10 weeks out (the expiration for Chrome pins post-build), and some of those pin lists get pretty long (CDNs, multiple CAs for whatever reason, multiple SPKIs per CA, etc).

TACK is designed to alleviate that pain for the site owner by providing flexibility, and by eliminating even the CAs the site uses from its scope of exposure.