You get 5 GB of free iCloud storage for every iCloud account you set up. It’s possible to use the same Apple ID on all Apple devices for the App Store and the iTunes Store, and a different iCloud ID for every person.
That way, you still have access to apps, music, movies, ... purchased with the Apple ID you use together.
Changing the iCloud ID affects everything tied to the ID: Photos, Safari bookmarks and passwords, notes, reminders, backups, .. The one with the new iCloud ID can’t use backups made with the old iCloud ID, and it would be necessary for the one still using the same ID to share the photos with the other one, just as an example.