They’re durable, resistant to corruption, relatively rare, pretty… Titanium or gems would’ve been equally used if they were equally convenient. Not to mention that coinage could be minted or mixed with, say, copper. Gold makes good alloys and can be recovered later.

> And why are/were they used as currency?

Because there was no useful alternative and/or we didn't know better.

Currency as jewelry is a good way to secure and carry your wealth as well as display it as a status symbol (or hide it).

A ring is just a coin you wear on your finger.