Blind and connected to Satellite Five feeds at the age of five by dozens of cables, the Controller acted as the eyes and ears of the Daleks. Aided by programmers, she constantly monitored the transmissions that flowed through her. (TV: Bad Wolf) Essentially, she served a function similar to the Editor's for the Jagrafess a century before, who also had occupied Floor 500 of the satellite, though the Controller had less say over her actions. (TV: The Long Game)
Nevertheless, the Controller loathed and plotted against the Daleks, ensuring that the Ninth Doctor and his companions would appear in game shows on Satellite Five (now called the Game Station), which the Daleks ignored. She waited until a rare solar flare temporarily disrupted communications between the Dalek fleet and her.
The Daleks could not monitor her thoughts, letting her warn the Ninth Doctor of the enemy "waiting in the dark". She then passed on to him the co-ordinates of Rose Tyler, enabling him to rescue her, though she did not at that point reveal to the Doctor and his allies the identity of her "masters".
The Controller told him part of the coordinates (5.6.1.434. sigma 777) before she was transmatted to the main Dalek saucer. Though well aware of her impending death, the Controller gloated that it made little difference, since she had brought "their destruction". With that, the Daleks exterminated her, but she was proven right when the Bad Wolf entity eradicated the entire Dalek force using the awesome and terrible power of the Time Vortex. (TV: Bad Wolf)
When Davros made the comment, "How many more have died in your name?", to the Tenth Doctor, he remembered the Controller as one of the people he was unable to save. (TV: Journey's End)