There need not be a specific end application in mind when you start to do such things. Its totally plausible that people who are inquisitive and have time, do things like this. Sometimes other people can find an end application. History of tech shows it very clearly..
classic ex: Xerox engineers made first GUI and didnt know what to do with it. It took a Steve Jobs, to connect the dots between GUI and personal computing.
Great article - i read through the build instructions but didn't see anything about the accuracy requirements for the application, but the fact that he chose a stepper motor leads me to think they were not that stringent, or possibly he has instituted some kind of microstepping scheme. Details, we want details!
Like any hobby this has its points of pride. Often people brag about connecting to difficult or distant satellites. While his is receive-only, he has quite the display to show off. I imagine there's a certain thrill in being able to "see" these things that are often quite literally invisible to the lay-person. The more you can see, the more respect you get.
This is all conjecture based off of my familarity with other radio enthusiasts though.
There's a little more insight in this link, where they discuss the joys of snooping the feeds from satellites. They can get a raw, unedited feed and see "behind the scenes". In the hacker culture, simply aquiring difficult to find information usually enough of a drive to get some amazing things done.
I don't have any exact details on that, however, keep in mind that he started with a system designed for this purpose(fine tracking). There could be gearing in place or something to refine the movement.