The function of a “tactical knife” ranges the gamut between combat and utility, generally incorporating some elements of both. Tactical knives are designed for extensive multi-use in the field, and the variety of such use is sparked by necessity while having little limitation. Specific tasks may include: paring fruit, trimming bark, cutting branches, skinning or scaling game, prying objects loose, cutting rope or cord, digging in the dirt, filching debris out of wounds, etc. (Note that some of these activities may damage your edge and might be better handled by more specific tools, but if a tactical knife is all you got, that’s what you’re going to use.) Finally, although tactical knives aren’t combat knives in that they’re designed for lethal use, they can certainly provide that in a pinch.
The big tactical knives are more cumbersome, but they have a couple advantages over smaller (folding) knives. First, the blade tends to be stronger. This is because it is a fixed blade, with a tang extending all the way into the hilt. (In prior years it was fashionable to have hollow-handled “survival knives”. While the handles may have contained all manner of useful goodies, they made the blade much weaker, with a tendency to break off at the base just when you were exerting the most force.) Second, the greater length creates more of a surface for sawing motions, if you have enough serration to perform that. Third, the blade width tends to be greater, which again gives greater strength along the cutting axis.
Knives are one of those things where you often get what you pay for. If you want a blade with real strength, look for steel that has a good carbon content. The right mixture of carbon and iron is crucial to providing proper strength and flexibility. You want your knife to be hard enough to endure the work, but flexible enough so it doesn’t snap under pressure. Also, since a sharper edge tends to get damaged faster, you want enough hardness to allow your blade to be decently sharp while holding its edge for repeated use. In general, avoid the cheaper stainless steels, as they tend to be more brittle. Aside from obviously ruining your tool, ill-timed snaps can actually be dangerous if fragile eyes or fingers are close by. There are all sorts of modern steels out there, but something like 1095 Hi-Carbon is a good combo of quality and affordability. Lastly, choose something with a good grip, like leather or overlay. When hands get sweaty, a solid grip is a must.