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

[Halivel]

Garland Tools shows both sources where you can get the item and where you can use it for most items in the game (they stopped to list armor and minions gotten from dungeons since Stormblood for some reason, though). Along with this one, there's "gamer escape" and "ffxiv consolegames wiki" that have a lot of info.

There also was a very good and deep database called ffxivdb, but they stopped updating it not long before the end of Stormblood and eventually closed the website, too.

And last but not the least is the official database that has almost every possible info you would potentionally need.