According to Dorium Maldovar, still points in time made it easier to create fixed points in time. The Kovarian Chapter of the Silence used one such point in space-time — 5:02 PM on 22 April 2011 by Lake Silencio — to make the Eleventh Doctor's supposed death there a fixed point.
Instead, the Silence merely assumed the Doctor died at Lake Silencio without realising he could and would find a way to fake his death thanks to the time-travelling vehicle the Teselecta, allowing him to survive. (TV: The Wedding of River Song)