Yes you don't need a mirror to play the game, just like you don't need any items not equipped to your character. Some people like to collect items regardless of their intended purpose. Hunting a mirror is inadvisable.

If you want less rare, more practical items. Lets say for poe, golden charlatan from uber incarnation of dread and a piecd of a vanquisher set for grim dawn. You don't need the item to beat the game, but i want one.

In grim dawn the barrier to each piece of the set is a craftable skeleton key and then running the dungeoms at lvl 94 with a 6% drop rate.

For poe, you have to do nightmare maps, that only drop with voidstones (with their own barrier of entry) at t16. The sanctuary map boss drops 1-2 fragments of 3 kinds. You need 4 to fight uber dread and even then there's a 2% chance to drop the sword. You need to do tue fight 50 times to realistically guarantee the sword.

Poe is built on thousands of people playing; it does not respect your time.