Why don't I find a site on Internet that explains what items are for?

[Rongway]

What you can trade it in for and where.

I don't really count that, as the answer to "what can I do with this?" is really just the answer to "where do I get this other thing?".

At any rate, between Garland, Gamerescape, and Lodestone, I'm not sure what kind of info the OP wants. That's pretty comprehensive coverage there. Yeah there's occasionally an item entry with a missing source, but it's a far cry from the dearth of information the OP suggests.