1) it's handy. That's like the overhead needed to write hello-world in C++. For simple jobs, the overhead is more than the job itself.
2) regular tools are very specialized. A multi-tool swiss-army-knife might have something that's good enough. But if using a good (expensive) screwdriver I don't want to use the wrong one! I might need several box wrenches to figure out which one I need, when an adjustible wrench (the VB of tools) would work.
Now some tools are general without being toys. A good strapwrench, a "gator grip", vise grips, a 6-tip ratcheting screwdriver, a dremel tool.
But, in the garrage, I see nothing wrong with having more tools! There is a big benifit to using the right tool for the job, and figuring out how to use a specialized tool is often easier than getting the job done without it.
I think that last point changes everything. Tools or accessories that are too much trouble don't get used. They naturally find their way to the back of the shelf and are eventually forgotten.
As for maintainance, that's not an issue with building things, but can be for household repairs. If a plumber fixed my sink, I might need to go out and get the same kind of tool he used next time I work on it myself. As it is, I know that anything I put together I have to right tool to take apart again. The first step to upgrading a Tivo digital video recorder is "go buy Torx #10 and #20 screwdrivers".
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