Useful for emergency release from a line entanglement, or a scenario where you need to cutaway from your harness or wing to prevent injury: such as getting dragged by a wing into hazards like vehicular traffic, or into water, or over a cliff, or through rough terrain.
Other potential problems might include water immersion, or a conflict between your wing and your reserve chute.
A nice selection of PPG line cutters can be found here at ParaGear.com:
Useful kit, but purpose designed knives and cutters for the skydiving/PPG/PG industry are priced at a premium, despite most of these tools being essentially the same as those made for other purposes.
They are also sold in specialty scuba stores as line cutters, at specialty rates, because of the limited nature of specialty markets.
The price comes down considerably, however, when sold in the Business/Industrial community as band or strap cutters, or for tools to strip the plastic wrap off of sealed pallets of shipped goods.
Selling them to fire and police personell as seatbelt cutters brings the volume up, and the price down, but specialty designed seatbelt cutters tend to have narrow mouth openings, and will only grab one or two lines at once, and make slower work of cutting away a paraglider, or reserve chute.
Despite working in EMS/Fire Rescue, I've never been fond of the all metal bladed hook knives, because I've never been convinced that a steel blade will outperform a clean, unused razor.
If a user succumbs to the temptation to test cut even one line sample with his expensive Gerbing or BenchMade hook knife, he will have degraded its potential usability in a future emergency, as these knives can't be sharpened by their owners. I heard that Gerbings will resharpen their blades for free, if you mail them in, but the number of lines that a newly sharpened knife of this type will cut is limited. It's also true that the number of lines that a razor will cut is limited too, but I'm betting the razor will out cut a steel hook like the one below.
I know some people will disagree. My view is purely a personal prejudice, and based only on the experience of one Swift Water Rescue Tech I spoke to who had trouble cutting some 8mm hemp rope.
The best, and cheapest substitute I've found is this one, fur worker's tool called a tail skinnner.
It has all the features of a PPG line cutter witht the added advantage of a wide mouth to catch a large number of lines into the razor area. Confirm the length of the tool you that purchase, as some of these tools are quite small, in the 3.5" range for example, and may not be ideal for people who wearing gloves, or with large hands.