For many reasons besides the obvious - to cut things whether it be in the shop, on the farm, fishing, in the bush etc.
I have always carried a knife - as a kid in the scouts we were allowed to carry a sheath knife even to school if you had your uniform on. In the bush when I started work it was expected of you to carry a knife - everyone did.
Now being a knifemaker and owning a knife shop it is expected of me to carry a knife and I still need it for the same reasons I did when I was younger - to cut things.
Looking at it philosophically there is something primitive about the knife - it must be in our genes (and jeans)- its hidden in our memory somewhere. As a knife seller it is obvious that knives spark something in certain types of people, whether they be male, female, the poor or the rich, the roadworker or the accountant. These people just like knives and like to carry knives. 90% of them don't mention self-defence or survivalism - they say they just like knives and many of 'em can't tell you why. Maybe it goes back our genetic inherentance or makeup. Maybe it is about taking the elements of earth (ore), fire, water and air and creating that most beautiful of objects - THE KNIFE.