Microlensing by stars within the lensing galaxy provides a way for astronomers to study the galaxies stellar content..
The focusing of the light from the quasar allows astronomers to observe individual stars in the quasar.
Lensing of quasars produces multiple images. One of the images may be delayed in time due to the different path length that the light has traveled. If an observation of an interesting event is missed with the more recent image, a second chance to make the observation is available.
Lensing tends to amplify the light of the quasar, making it easier to observe.
By observing the time delay between different images of the quasar, it is possible to determine the distance to the lensing galaxy.