Why does it say my pictures will be deleted from my icloud when i try to delete them off my phone, in that case what is the point of purchasing extra monthly storage or even having an icloud in the first place if my pictures will be deleted?!

iCloud Photo Library is not designed to be a cloud-only storage service. The photos kept in iCloud Photo Library are also on all of the devices where you have turned iCloud Photo Library on and any photos added to or deleted from one device are automatically synced to iCloud Photo Library and to the other devices. You can reduce the storage space required on your phone (and also on any other devices) by going to Settings > Photos & Camera and placing a checkmark in front of Optimize Storage. More details in iCloud Photo Library FAQ - Apple Support

Apple does not offer any cloud-only storage options. If that is a requirement there are 3rd parties that do. You could also make the suggestion to Apple at https://www.apple.com/feedback/icloud.html