Because upvoting a story that seems interesting (ie: Yelp files for IPO) and wanting to save a submission related to some new service or project (ie: ClutchIO or some github project) are 2 different things for me. I never want to go back and read about Yelp IPO but I may in the future want to use that service or project submitted. That's just my use case though. I use read-later service now to do that.