Sufficiently large asteroids could create massive amounts of damage when they hit a planet, causing mass extinctions and societal collapse. Wherever they landed, all the surroundings would be incinerated within seconds. The impact would kick up a cloud of dust which could cover the planet and cause an "Impact Winter" for months or years. Burning debris and acid would happen. Following the "Impact Winter" was the "Ultraviolet Spring", which would cause mutations and cancer. (AUDIO: Blood of the Daleks)
As recounted by K9 Mark 2, the civilisation on the planet Ceres was wiped out by an asteroid, the approach of which they were oblivious to due to the symbiotic, two-class Master/Scribe system, which was created with the substance cerulium. (TV: Sirens of Ceres)
While many asteroids were barren and lifeless, some supported life, either naturally or due to design.
A wide variety of buildings and structures were built into asteroids, including the Bi-Al Foundation, (TV: The Invisible Enemy) the Braxiatel Collection, (PROSE: Tears of the Oracle) the Delirium Archive, (TV: The Time of Angels) Demons Run, (TV: A Good Man Goes to War) Lavonia, (AUDIO: Fear of the Daleks) the Shadow Proclamation headquarters (TV: The Stolen Earth) and the Stormlight House. (COMIC: Mortal Beloved) Other asteroids were centres of trade, such as one on which the rare metal Bazoolium was sold. (TV: Army of Ghosts) They generally needed special features to bring the gravity to acceptable levels. (PROSE: Tears of the Oracle)
At the end of the universe, the Cyberiad lived in an asteroid, one which had been moulded into the likeness of a Cyberman head. Following his exile, Rassilon visited this asteroid to propose an alliance, being welcomed and incorporated into the Cyberiad as the Cyber-President. (COMIC: Supremacy of the Cybermen)
Usage of asteroids
Upon taking Rose Tyler to see the Earth's destruction in the year five billion, the Ninth Doctor mentioned asteroids as one of the things humanity thought would cause their extinction. (TV: The End of the World)
Asteroid 284996 was named Rosaparks in honour of civil rights activist Rosa Parks. The Thirteenth Doctor took her companions to see the asteroid to show them Parks' lasting impact on the universe. (TV: Rosa)