When you are doing research, you tend to collect a lot of papers. I remember that at the end of m PhD, when I moved to another continent to do a postdoc, I dumped a huge box of photocopies in my parents’ basement. A few years ago, I had collected two cupboards full of photocopies. It was getting seriously out of hand. Then, of course, journals started putting everything online as PDFs and the same process started all over again but this time filling up hard disk folders instead. I used to have subject-based folders, which sort of worked until something fit within 2 or 3 or 4 of my subjects. Searching for some old paper you had read a few years back became more and more nightmarish. Then somebody showed me Papers.