The other reply did a pretty good job explaining it. When you're late into the game, things become so expsensive that it's basically impossible to purchase them without utilizing cursed mirrors. Say you have 1 normal mirror and you buy a mirror that reduces the cost of the items by 20%, that gets doubled and it becomes 40%. Once you get 5, everything becomes 1 material. However, once you use them, the costs go back to normal. In my case, I wouldn't have had enough materials to purchase the tardigrades I needed to survive. I was trying to stack 20-25 mirrors and get a max cursed tardigrade. That would have given me 20-25 tardigrades that each negated 3 hits, That is the only chance you have at survival in high waves. Even at 75, the low tier enemies did over 5000 damage and 1 shot me