They are used in last-generation telescopes, to correct the aberration introduced by the atmosphere on the light path and improve image quality over what would be possible according to the night seeing.
A significant fraction of the aberration results in simply moving the image by a small amount, in a random direction that changes several times a second. A tip-tilt mirror is introduced in the optical path of the telescope, and is moved in realtime to reposition the image in its theoretical center. A fast detector is needed to achieve good results.