1. The official use is as a package hook, to turn your knife into a handle that will help you to carry a parcel that is wrapped in string.
2. Similarly, you can use to carry heavy plastic bags. Normally when a plastic bag is heavy, the handles become thin and carrying it for a long distance will hurt your fingers since the plastic handle will dig into your fingers. You can use the hook to grab the heavy plastic bag.
3. On my compact I can use it to hold the pen when writing. It is much more comfortable to use that way. Refer to this YouTube Video
4. As a fishing aid it is extremely useful. You can hook it through a fish's jaw and carry the fish like a strung parcel.
5. It can also be used for grabbing wire wrapped hay bales, so you don't cut into yourself.
6. You can also hook the bail of a cook pot to lift it on or off a camp fire.
7. Check also this YouTube Video
8. Additionally, on my Compact knife it also has a nail file on its back. Refer to the image below (bottom hook).
If anyone else knows of another practical use, you are welcome to build on this discussion...