People have lots of photos/videos, and when your phone runs out of space, it kindly prompts you to buy iCloud storage. I have several family members who have done this, without understanding what it really meant.
Also note that photos/videos taken on new phones are much, much larger than on older phones, effectively meaning that you can store way fewer photos/videos than before.
(This is not a defence of Apple btw - it's unforgivable that they lock down the phone so you can only use their backup feature, and it doesn't actually work).