Unfortunately, living in a modern suburban society, there is a decreasing need for much of a pocket knife. Fortunately, we live in a very very prepackaged society, and that means a sharp knife is needed pretty often if you do any kind of shopping at all. The inventor of the cursed plastic blister package must have been a knife knut, because that's the easiest way to open one of those d--m things. Camping and hiking, and fishing certainly call for a knife, but modern life in suburbia also needs one. There's UPS and FEDEX boxes that get dropped at the door, especially if your better half likes to shop via Amazon and other online outlets.
Then there's grilling. If I've got something on the grill for dinner, and I want to make sure it's done, I could walk back in the house for a knife, but it's far easier to just take out my pocket knife and make a slit to check to see if it's done just right.
I don't like gardening or landscaping, but being a home owner it's a necessary evil. Lugging a couple of 50 pound bags of mulch from the back of the pickup around the house, I don't feel like going to look for a cutting tool. Easy to just reach in pocket, open knife, cut bag open, close and drop knife back in pocket.
And then there's father time. He gets us all in the end, and he will rob you of the youth and strength. As a senior citizen, I found that with some arthritis in my fingers, it's way way easier to just cut something open that try to twist, tear, rip it open like a young man can. I've finally reached an age where opening a package can be a major PITA, so I just take out my nice sharp knife and cut the darn thing open.
But all this is aside from the fact that sometimes, not very often, you may be confronted with an emergency. No, I'm not talking about Chinese paratroopers or zombi hordes. No, a real world emergency where you, or maybe someone you don't even know, needs help right now, with no time to spare. It happens. Twice in my life, I needed a shape knife desperately to help someone in trouble. Because of a life long habit, I had a sharp knife in my pocket, and was able to help. It felt rewarding.
But most of all, you carry a knife because you can, and you never really know what you will encounter in the course of your day before you make it home that night. And sometimes, maybe just once or twice in a lifetime, yours or someone you may not even know, will have a life hanging on someone having a sharp knife on hand. It doesn't have to be very big to cut a seat belt, or a scarf caught in an escalator that is chocking someone. But it does have to be sharp.