Just one use case:
Being on the way with a mobile phone, gathering information, by bookmarking links and saving web pages for offline reading. Later it becomes of interest to go through all that, but now you have a tablet at hand. Bigger sized and more convenient for relaxed reading. You wanna quickly get all the freshly (and only these) bookmarked links onto the tablet, to quickly look them through and save the one or other site for offline use as well. Then you enter the plane, and being online becomes the past, for some hours. Now you have time to move also all the latest offline pages from your mobile phone to the tablet, for relaxed rading, and you have time for that as well...