What are runes for?

[Boo Seta]

I have some of them, everytime I can forge better ones I do, but I have no idea what's the purpose of them. Could someone explain, please?

[chaoticenigma]

Outside of getting the achievement for forging them, you should never forge them. 3 for 1 higher one that you can't even pick isn't worth it.

Runes are used for activating shrines for buffs during that run, tattoos for benefits for the rest of that run, and activating a runeword for your next run. You can also use them for certain events. A good one is Savior's Brotherhood event where you can trade all your Copper/Silver/Gold runes for runes of a different type, and with how common Copper ones are, you can trade them for Silvers or Golds at random.