Putting something into your Safari Reading List saves it for offline access. If you want to read a webpage later on mobile (and it won't parse through Instapaper/Pocket for some reason), that's probably the way to go, vs. leaving it open as a tab.

Though, admittedly, I never got it to work right at actually using the offline-snapshot versions of pages when I was, er, offline. Even though it clearly had them taking up space on the disk.