I heard lots of times that designers tend to do that.
Why is it good to separate analog and digital components and grounds from each other?
After all, the whole board has a single ground, so what does it mean to separate grounds, if they are the same?
Two separate ground planes means that you have two ground planes: digital and analog, that are connected only at one point.
What it does is the analog signals and digital signals return currents don't cross paths in the ground plane unless they have to. It's mainly so noise in the digital plane from switching doesn't contaminate the more sensitive analog ground plane the problem. THere's a catch though...you have to properly route the signals that do have a return path that will cross between the ground planes. If you don't route the signals properly, the current loop formed will be large and cause more noise than if you just had a single ground plane to begin with (the best return current path being the path that lays directly beneath the signal path thereby forming the smallest possible loop). With a single ground plane, it just takes the shortest return path.
dknguyen, interestingly the article in the link you provided argues that, for many cases, a well designed single ground plane is preferable to separate analog and digital plane.