The process is known as lucky imaging where a high frame rate video is taken to provide thousands of frames in a short period of time. There are a few reasons for this. One is atmospheric distortions. The atmosphere is a very dynamic fluid that can make it look like the planet is underwater. However there will be brief moments of clarity so the more frames the more of these moments that are captured. You then stack these frames on top of each other in order to get a clearer photo. Doing this will also improve the signal to noise ratio and will reduce the camera noise (grain).