Primarily, self defense. Secondarily, just to use to cut open a letter, box or whatever. Carry a karambit for the former and a folder for the latter; both at the same time.
In fact, I just used my karambit in justifyable SD this evening. Drunk & disorderly and battery charges filed against the assailant, who was arrested.
Guy was itching for a fight. Was minding my own business, eating a salad and drinking a beer at a local sports bar while watching the post-game interviews after the Niners game. He sat next to me and pushed against me. I moved and asked if he had enough room. He moved in even closer and asked me if I wanted to fight. Told him no, I didn't want to fight and wished him a Happy Halloween. He then apologized and appeared to let it go but then I called 911 to report him as a possible "drunk and disorderly" person under PC 647(f).
Was actually on the phone w/local 911 when I was attacked and that was probably why I was attacked. Knocked me on my ass off a tall stool seat and the phone out of my hand but as I fell I instinctively drew my small Bastinelli/Marcaida Pika karambit. It only has a 1" blade but it's razor sharp and still cut him w/it thru the heavy jacket that he was wearing. No time for any fancy blocks or counter moves; could have killed him if I did.
No charges filed against me because I was in reasonable fear of "great bodily injury" as a result of the attack. The assailant weighed at least 250# and only weigh 150. They also have a 911 tape of the call and incident while it was in progress.
PD arrived just a few mins after I was attacked. ID'd myself as a retired LEO and described my side of the incident. Had a least 5 witnesses (including another retired LEO who was known to some of the responding officers) who supported my version of the incident.
The knife was confiscated as evidence as I expected would happen. Took awhile for the supervising officer to ask for it. Didn't volunteer it but gave it up promptly when he asked for it. I expect it to be returned to me eventually (who knows when?) but I have another. Just be aware that if you use a weapon (any weapon) in SD or otherwise, that it will automatically be seized as evidence. So, don't ever carry your favorite knife (or gun) as your EDC.
Fell directly on my ass on a hard floor. My ass hurts more now that it did immediately after the incident; probably because I'm now coming down off of the adrenaline rush. By coincidence, I have a doctor's appt tomorrow and, if still hurts then, I'm going to ask for x-rays of my pelvis (and my left elbow which also hurts) and report any fractures to the supervising officer.
PS: Some will probably say that it would have been better if I carried a gun (which can do legally) but a gun would have not prevented the attack because it was so sudden and he was so close to me and, even if I had drawn a gun after falling to the floor, it would have probably just resulted in chaos (even after I ID's myself as a retired LEO). It would have been even worse if I actually shot him in response and, even if I drew the gun but didn't shoot him, they probably would still have confiscated my gun.
So, better a knife in this and similar situations IMO.
PPS: I was also carrying a 4" long Spyderco fluted Ti Millie at the time but there was not enough time to draw it and I would have hated to had that confiscated in the process because it is a very special knife. However, this incident just goes to prove (at least to me) that if you are going to carry a knife for SD, better to carry a fixed blade knife for that purpose than a folder.
BTW, there was no discussion of whether I was legally carrying the karambit -- ie., whether it was concealed or not -- before I deployed it. Hence, no such charges were filed against me and no way any such charges can be filed against me after the fact, unless I admit to concealed carry which of course I would not do.