What needs tin plates? I have found no object that needs tin plates.

okay Large power pole and bronze plate(or bronze alloy) needs tin plates(for example), but few objects need that, or have I overlooked so many?

Yeah, Tin as a construction material is mostly just used to make Brass and Bronze.
Brass itself isn't used all that much, it's mostly used for the brass bearings and pipes, during T3 alongside aluminium.

The actual big use for this is in Electronics, as it's used to make Solder, and Tinned copper wire, which is a component for most electronics components, but that requires having the Electronics mod installed.