The "recent list" only lists threads you have not opened for view, that are three months old or newer (calculated from the date in the original post--a major design flaw in my opinion). As you open and view threads, they disappear from the Recent list, but remain in the All list.
Thus, especially for regular users, this is a very useful way to determine which threads have not yet been opened for view. It can keep you from skipping a thread you might have wanted to read and from opening the same thread again when nothing new has been posted to it since your last time viewing that thread.
Thank you Phil for explaining the difference between both lists.
I guess, I had a hard time to recognize the special feature of the top list because I often open multiple threads in a new tab in my browser, instead of returning to the main page after reading a post.
Perhaps FileMaker should rename the top list: "New FileMaker Pro Posts".