Noob question about item bases

Relatively new in poe but I've always been confused with item bases. I was hoping I could be enlightened on.

Things I do know: certain mods can only roll at certain ilvl on gear and sometimes its best not to go too high on ilvl for gear because you open up the possibility of being exposed to additional unwanted mods that could make it difficult to craft gear.

Things I'm unclear about:Why are lower/weaker item bases used by people if ilvl is the same? Example: A guide will suggest to use vaal gauntlets for a strength/armor based build but why not any of the higher str/armor bases above it assuming ilvl is the same? Wouldn't having the higher base simply provide higher armor?

In the same context to my previous question, ill notice people using varying evasion,armor,es mixed item bases even though their build might be more focused on a single defense stat, is this simply the only item they could buy from trade?

Just trying to piece it all together thank you!