Let us say that 80% of the time, I use a hammer to knock nails into wood. But 20% of the time I use screws. I need a screwdriver then, rather than a hammer. And then, 1% of the time, when I am dealing with nuts and bolts, I need spanners (wrenches in American) or socket sets. I have quite a few tools that I might use only two or three times a decade -- devices for extracting broken bolts, for example, or for removing nuts with rounded heads -- but when I do use them, they are essential. Likewise, if I am shooting a portrait, I know that for one style I'll need my Thambar; for another, a 90 Summicron; and for another, my 8x10 De Vere with the 21 inch Ross.
Cheers,
R.