Photo storage

I am having huge photo storage issues. I have an iphone 6S with 16GB of storage. I have my icloud set up. I paid for extra storage n my icloud so I now have 50.

I thought that I could free up storage space on my phone by deleting photos from my phone once they were synced to the cloud but have now found out that it has deleted them on the cloud as well.

What is the point of buying extra storage on the cloud if I can't keep photos on the cloud when I delete them from my phone?

I know I must be missing something, please help, I want to know how to transfer photos to my icloud and keep them safe there while deleting them from my iphone so that I have enough space to take more.