They are particularly important in fly-by missions such as New Horizons, which has been sending back stunning images of Pluto and its moon Charon.
With a flyby mission you can’t probe the ice on the surface or analyze samples, so you have other methods to determine a planet’s makeup and age – and gain insight into how it involved into the world it is today. Analyzing craters can help us understand the age of a planet’s surface.
The first thing they can tell is age. A surface with more craters indicates that it’s geologically older than a smoother surface, although that isn’t always straightforward.
Age-dating based on craters is complicated by a number of other factors as well. I will highlight one big issue here: How one “sees” craters is affected by lighting over the planet’s surface. Just like on Earth, the lighting on Pluto and Charon changes both with latitude and over the course of the day. When the sun is directly overhead, there are very few shadows cast and it is hard to see topography, but that overhead lighting makes it much easier to see dark or bright markings. The opposite is true when the sun hits the surface at a shallow angle near sunrise or sunset: topography is easy to see, but bright and dark colorations are often washed out.
Because New Horizons flew by Pluto so quickly, the highest-resolution pictures were all taken under the same lighting condition and so northern latitudes have the sun mostly overhead, while near the equator the sun hits the surface at an angle.