It's a reasonable measure of surface gravity, which, along with an idea about the distance from the star it's orbiting, gives us a good idea about what kind of atmosphere we can expect (i.e. the lighter and the hotter a gas is, the easier it can escape a planet with a particular escape velocity), at least a basic idea about what kind of surface features we might expect and so on.

It's also a parameter that is quick to compute starting from radius and mass, which are often the first things you know about an exoplanet due to the way they're found.