That's not how photos work on any of my Androids. The original files are on the phone, and if I need to make space I can move them to a computer via USB (or dropbox, email, etc.).
You must have the Google photos sync option (or something like that) enabled.
Thus there certainly is a reason to do this. Its how photos work on iphones and androids as well. You take pictures, they are put on the cloud, space can be freed up. Its not a malicious workflow