Understanding that I am a Scout Master, an avid backpacker and camper, and do some of these activities for work, including teaching wilderness survival, I use my knife frequently. I generally carry three: a sheath knife or large folder, a Swiss Army Knife (the Ranger being my preferred model for the saw, leather working tools and metal file/saw), and a multi-tool. If I am going light, I may swap the regular multi-tool for a small one like the Squirt. How do I use my knife/knives?

Wood working (whittling, shaping for pioneering projects, cutting improvised tent stakes, primitive technology crafts, cutting rope and string, cutting wood for fire or shelter)

Food prep, especially fish and game

Repairing gear (sometimes you have to cut to cure)

Lighting fires (in conjunction with a Ferro Rod)

Digging

Clearing a spot for my tent or bed

Building stuff out of plastic or metal that is found (reiterating what Bacpacjac said)

I will also generally carry EMT shears, and encourage my scouts to do it as well (especially the younger ones). They will perform many cutting jobs, are one hand operation and entirely safe.

I am generally not too worried about a little extra weight. I also carry a pocket sharpener,I like the Smith's as it has the carbide, ceramic and a diamond rod. The carbide is only for repairing major damage to the edge as it removes a good bit of steel. I tend to use the ceramic frequently to maintain the edge and the diamond rod if the edge needs more work.