Allow mark all as unread
When I subscribe to a new blog, older entries are marked read. Often I want to browse the entire feed history, and currently marking all unread is quite tedious.
Jamie A commented
Because you subscribe to a new blog, you might want to read through the old posts. I normally go through and mark all the old posts I can as unread so later I can go through and read them.
Interesting, why you're trying to mark older items as unread? To browse history you could just select "Show all" option in drop-down menu below search ("N New" button).