Well the two similar functions of a knife are stabbing and cutting. Both of these actions can be applied to items to modify and improve their function to my advantage. Probably the biggest application is opening a container like a shipping box or mail. Another common use is in food preparation where I cut away undesired pieces and modify the shape and size of my food for better cooking and consumption. Another would be modifying a tool to function better like sharpening a stick so I can stick it in someone's foot and so not get blood on my knife. Then there are situations where I don't have time to undo someone's bindings but with a knife it can be accomplished quickly and sometimes with one hand while my other hand can be used to push them off the bridge or over the cliff. Other random applications that come to mind, car accident where I need to release someone from their seatbelt, enforcing discipline with my children as in, "you kids better get to bed right now or so help me I'll skin this puppy right now!", financial negotiations, "could I have some money please?" Saving someone from damnation by rendering his inflatable doll unusable. Getting to second base(those damn clasps!). Sometimes it is important to appear to be the crazy person in the relationship, "hey mother-in-law its great to see you, its been so long since your last visit" Getting a date with a coworker, "oh wait Mary let me help you open that UPS box. Yes, see how easy that was. Hey, by the way......"