tom, unless you check FR frequently, posts can "age off" the board. On a really busy day, that could be just a matter of hours. Your ping list doesn't age off, so if you are pinged when the dose goes up, you won't ever miss it. You will always see it when you check your pings.
It's insurance to make sure that you won't miss a dose or an update. Esp. the times when there are few pics so our wonderful dosers just add a few to the previous day's post. Those get pinged too.
I don't like to miss any, and the ping list is the best way to make sure you don't.