Because it can take a while for a page to open whereas I can zip through and open twenty and during the day as I have time to read I go to a tab and start reading. Sometime I have had enough of a subject for now, but may want to come back and continue at the same place I was reading, I can flip to another of my open tabs and start a new subject. Close all tabs and I've read all the slashdot stories for today. Rarely do I want to come back once I've finished reading so book marking isn't really the right model. I get it, it's not the way you want to work but it's efficient for me.