Specialty tools that fit vehicles you don't have. Specialty tools for operations that no one would perform by hand anymore. E.g. a cylinder honing tool.

Whitworth wrenches, sockets, taps, and dies. My dad had a British motorcycle shop around 1980 and I'm pretty sure there is still some of this in his tool boxes in the garage.

Homemade jigs whose purpose you can't even figure out.

I think the dominant category would just be extra or broken or cheap tools that aren't worth hanging on to. For some reason I have like four stubby Phillips #1 screwdrivers. Cheap hatchets with broken handles. Mushroomed lead mallets. A whole drawer of dull drill bits.