Knives are generally useful, but not specifically useful. They are often a good substitute for a more appropriate tool. One can use a knife as a can opener, or to cut down a sapling, or to dig a foreign object out of a wound. It may be that a knife is the most generally useful tool of all time: if one can only have one tool, it should almost certainly be a knife. Part of that generality includes misuse, and people tend to have more specifically-useful tools (such as can openers, saws, and surgical implements), which tend to be both less useful and less misuseful (because that's a perfectly cromulent word, no matter what spellcheck says).