Here’s a description from the projects CodePlex page:
Say you have a Site Collection consisting of i.e. 60 subsites, and you want to add/remove/update a site feature on each and every site.
Out of the box, SharePoint will not let you activate/deactivate such Features in one operation. You simply have to go into each site and activate/deactivate the Feature manually. This is very time consuming…
Some might argue and say that a Site Collection Feature in some cases would be better, but what if this does not suit your purpose.
"What if I could just select a Feature and simply activate/deactive this on multiple sites/site collections in one operation…?"
You can find more information on the Balle on MOSS blog.
The projects CodePlex page is here.