I have a large collection of random screws, but I can never seem to find the one I need when one goes missing. Have you ever tried buying a screw by matching all its characteristics? Obviously you need to know how long it is and how big around it is, and whether you want Phillips or slotted. Do you want a point on the end? How about the lag between the head and the threads? Of course, you need to know the threads per inch and the thread depth, especially if you are using the screw in a metal object. The head of the screw can be flat, oval, pan, truss, or hex washer, and I'll be darned if I understand the advantage of any but the flat head. I particularly like this website, which even has a section for "sex bolts and mating screws," which seems appropriate because of all the double entendres possible when dealing with this form of hardware.
So many choices, and all of them dirt cheap. I keep old screws because I have a hard time throwing anything out, and because they're so small that they're easy to store. I'm not sure I've ever used any of these around the house, however. There are so many different sizes that I usually can't find what I need, even if I happen to have the right combination of length, diameter, thread count, thread depth, and so forth. The thing is, there really isn't any point to keeping screws. They are so cheap that it would certainly be no imposition to pay for the exact kind I need every time. In fact, it would be cheaper if you count in the time I spend looking through every one of six dozen different screw types trying to locate the one that fits.
Screws are a very humble part of the modern industrial age, although doubtless many items in my house would fall apart without them. When I first learned about simple tools, and heard that screws were a variation on the inclined plane, I didn't understand it at all. Now I can see how the "threads" -- really a single thread -- is just an inclined plane that converts circular to linear motion. The inclined plane hardly seems like a tool at all, but it lies at the foundation of so many mechanical items: not only the screw but also the ramp, the wedge, the knife, and scissors.
I like screws because they are easy to use -- I have no talent for hammering nails (another use of the inclined plane, by the way). I have heard people complain about Phillips head screws; The Straight Dope says that the advantages are chiefly for industrial applications but not for individuals. I don't know how anyone could think that, since I have spent many frustrating moments when a flat screwdriver slipped out of its slot and I had to realign it. Does everyone think that slot screws are better?
The one problem with Phillips screws is that it is so easy to deform the slot, making it impossible to remove it again. I have always wondered why Phillips screws are particularly susceptible to this, and how hard it would be to make a stronger screw that would not buckle so easily. I'm sure it would cost more, but I would be willing to pay some premium for a stronger screw; the question is, how much more would I have to pay?