Although the application can't do anything if the file didn't finish downloading (no application can), it can repair zipped files damaged by things like the computer freezing or suddenly shutting down, or the file transfer showing an error.
The process is pretty simple: choose the damaged file, take a look at what it has inside (it can take some time to display this), and then unzip the content. That easy and straightforward.
ZipRemedy is one of those apps that can never hurt to have on hand. It's not common for a zipped file to be damaged, but it's better to be safe than sorry.