What is the purpose of buying extra storage on iCloud if deleting photos from iPhone deletes them from the cloud too?

I am reminded that iPhone storage and iCloud storage are different. I wanted to save space on my phone and delete pictures thinking they will be safely stored in the iCloud for retrieval, I've learned this is not the case. So why do I pay for extra storage in iCloud to safely store items that I don't want to lose, if iCloud is just mirror of what I store on my phone? In other words, if my phone has XGB of storage but I purchase a larger quantity from iCloud because I want to store photos, why do I lose them when I manage my phone storage? What am I missing? Please educate me on the difference. Apple is deficient in basic knowledge to actual consumers and assumes everyone is in the IT business. Doesn't this mean that purchased extra iCloud storage is wasted because it just mirrors my phone anyway? Most people assume the cloud is a place to store reserves of items they want to retrieve later.