Because I like knives, and I can't decide on which one or two I want to carry on any given day.
Because I like to have a really sharp edge available all the time. If I have to use one (which I do frequently) and I don't have access to my sharpening gear, the next blade in the rotation is quickly available.
I carry the same way. My defensive folder in clipped to my strong side. I use a schrade old timer that was my grandfathers for small cutting jobs and I always have a 9 function swisss army knife for cutting and general utility.
I have a CS Tuff Lite Tanto on my keychain, which means it's pretty much on my person at all times. I also carry either a Sog Spec Elite 2 or a Flash 2 on my person everyday. In addition, both my vehicles have a knife stuck between the drivers seat and center console, a CS Roach Belly in the Buick and a Bushman in the Jeep. I'm not going to go into the ones I have stashed around my house within easy reach of where I spend most of my time
Usually 2-3. Leatherman of course, then my "good" knife which never ever ever gets handed to another person, then often a beater knife which if someone else screws up is no big deal.
I carry more than one knife. One needs different tools for different uses. I carry a small lockblade for small chores, a larger lockblade for bigger chores or defense, a geber boot knife for defense as well as a gerber tool which has two blades.
I carry a fixed blade knife on my belt as a back up to my gun. I have a leatherman because pliers are perhaps one of the most useful tools to have around, not to mention the other attachments and I carry a swiss army knife for the additional tools.
I dont carry my leatherman like I used to but even if I have it on my belt I always have one in my pocket as well. I use the Leatherman more for the tools than cutting.
I also keep a self defence knife in my truck along with my pistol.