Backup

No, the purpose of a backup is to safely store the contents of your phone so it can be restored at some time in the future.

There’s a complication; if you delete everything by going to Settings/General/Reset-Erase all content and settings, or if you restore the phone to factory conditions using your computer, then the backup plus anything synced to iCloud will restore everything.

However, if you manually delete content (such as photos) then anything you sync to iCloud will be deleted also. Note that I said “sync” to iCloud. iCloud has 2 unrelated functions: Backup to iCloud and Sync to iCloud. These are completely independent of each other; anything you sync will not be included in a backup. You can use both, but they do not overlap in function.

You can delete what was on your phone and the already made backup will not change HOWEVER the next backup will not include the deleted data so you could lose everything you deleted.