In the episode "The Waters of Mars" we're introduced to the character Captain Adelaide Brooke, who is an integral part of human history as her death on Mars in the year 2059 inspires her descendants to be a driving force in humanity's exploration of space.
We learn that a Dalek spared her when she was a child during their invasion of Earth in 2009, as it was aware of her nature as a fixed point at a future date.
However, during the Dalek's invasion their ultimate goal was to detonate the reality bomb, wiping out all realities except for themselves leaving them the only race in all of existence. With this in mind, why did the Dalek spare the young Adelaide Brooke? The Dalek's presumably planned to succeed in detonating their bomb, thereby making her future meaningless, so why spare her?