You get tangled in kelp or sea weed
You get caught in a fishing line or underwater fishing net
You are an international spy being attacked by enemies and need to cut their air hoses in a dramatic underwater fight.
Honestly, in my opinion, a dive knife is a frivolous gadget. Unless you are diving in an area where tangled kelp or dangling fishing lines are a possibility you do not need a dive knife. Most divers who strap knives to their legs (never to the arms - it can get caught on your hoses) are wearing just to look slightly more macho in their wet suit.
Nevertheless, if it's a dive knife you want you might as well get a good one. Here are some tips for choosing a dive knife:
See our choices for dive knives
Search for Scuba resources