The whole point of Google Photos is that it is a a cloud backup service that indexes a lifetime of photos and manages them for you. Google Photos without the free, unlimited storage misses the whole point of liberating you from having to ever worry about managing photos again.
Saying Apple Photos, the app, is free is like saying email clients are free. Sure, you could store a lifetime of email on your phone, but the original value proposition of Gmail was that they gave you so much free storage at a time when services like Yahoo and Hotmail! charged you for more than 25mb. Most of the value is managing the storage for you, worry free.
Don't give you users shitwork, and having to manage your phone's storage, sweat over metered storage, and delete stuff to free up space, is needless, janitorial, shitwork. That's why the web is so amazing, because I don't care about what websites I visit, since I don't have to manage what's in my browser's cache, it is purged automatically if unused, and anything important I do online is persisted in the cloud.
Storage management is annoying and anti-user, and Google Photos is about peace of mind.