A multi-tool is just the next incarnation of the venerable Swiss army knife. It's not perfect, but it can be used to improvise in many situations when it's simply not possible to carry a tool box on your person (especially one that so easily slips in a pocket). Instead of having to carry a separate locking blade, screwdrivers, file, saw, can opener, bottle opener, wire cutters, pliers, ect; you have everything in one easy to carry package.

For me, I normally carry a Benchmade 707, but that often gets switched out with a Leatherman Charge when I feel like I'll need it. I also have a Leatherman in my truck and in my backpack. They've come in so handy over the years it's an indispensable part of my gear. Snowmobiling, camping, boating, hiking, working, school.....every environment I've been in it has proved useful at one point or another. Same with my little surefire flashlight. That's why I always keep both nearby.