The internet is not always available for at least some people.

"Why not just slow down your device?"

Tabs aren't meant as bookmarks. Read later is for saving anything to any amount of time, and it doesn't take up responses of your system, is searchable, has tags, reading view etc. Your comment is grandma with dementia level of tech illiteracy.

The main idea is that you can access it regardless of which device you're currently using. Like saving an article you see when you're on your PC for when you're about to leave so you can read it on your phone while on the train