what is the reason/value for mirrored items

i am new to the game and not sure what the purpose of gathering mirrored items might be

Reason for many people to play PoE is to create best character. To create best character you need best(mirror worth) gear

Others play to be rich. To be rich you need best(mirror worth) gear.

They are some of the best items possible to get. Most RPG players like to improve their characters.

what i would like to know is what makes them the best

Well, for an item to be mirror worth it needs to have proper combination of affixes and prefixes on it. And even then it needs to be in top 1% or 2% of those rolls. You will find a lot of items that are complete garbage, but depending if you are playing standard or the new leagues some items are worth more than others. I'll get bashed but standard is pretty broken and there is no reason to play it other than testing new builds and stuff. The trade is filled with scammers and flippers so some items are WAY overpriced. Let me get back on point, Most items are going to be trash, but if you are playing the new league you are in luck. In the beginning of the league most items are sellable. Usually, the things you ask in gear for armor are high life rolls, duo or trio resistance and flat and % roll on the base that armor is (armor,evasion,energy shield or any hybrid of those). That is a dream roll defensive wise. IF you get those rolls for example, now you need to check if they are high enough. Life can be rolled basically from 10 to 120(aproxx) depending on the item level. You should probably check the mod compendium on the exact numbers. So getting every piece of mirror worthy items is a hard task.
Sorry for the wall of text but having mirror worthy items means that you can destroy any content out there ( if the build is good ofc). Also, having those items also mean that you could 'force' some builds to work which would not be possible if the items were of lower quality.
I hope some stuff are clearer now.

a mirror is an orb that simply duplicates a rare item, the 'mirrored' copy cannot be altered, you cant change the socket colours or modify the stats in any way. Its used to make copies of THE best rare items in the game. What makes a rare worth mirroring is that its literally the best possible rare item in a given slot for a given build, theres no item out there with closer to perfect stats. If someone has mirrored gear it basically means they have copies of some of the best rare items that exist.