After an app loses its location lock, it generally has to go through a few things to get it back. This is not so much an issue if you're in a town where seeing the sky is a given, but in places where it's steel and concrete as far as the eye can see, it's going to take a bit longer.
This is especially annoying if you've used your GPS navigation to get to a place, and then want to check in on Yelp and FourSquare and Facebook Places and Google Places so you can get badges/points/nerd cred. Each app potentially will have to start the process of getting the location all over again.
GpsFix has an answer for that. When turned on, it keeps a lock on your position for however many minutes you set it to so even if an application drops requests for where you are, your EVO will still actively be requesting updates. This is useful for when you want to use GPS but don't need to be running an app like Navigation with the screen on.
Unfortunately (but not surprisingly), GpsFix does drain your battery. The "fix" portion means it acquires a fix on your location while it's running, not that it "fixes" the way these programs operate. I use it only when I am in an area where I know I'm not going to have a lock and will need it in a timely fashion.
There are a limited number of scenarios where you would need this tool, but it's handy and free.
Thanks, VaelPak Forums!