It's just an outgrowth of the fact that I find commercial aviation fascinating, really. I'll occasionally pull up flights near where I am, but I usually use tools to track whatever flights I find interesting for other reasons (off the top of my head, for instance, American Airlines' first revenue 787 flight was today, and I watched that one this morning).

Some people enjoy tracking birds some people enjoy tracking aircraft. If you're not into that you probably won't get it I knew a guy who was a plane spotter and he would get a kick of being able to recognize a commercial jet by its sound and even estimate it's approach and altitude. I didn't get why he enjoyed that, and i don't think that you can relate to that to understand it if you didn't decide to take up that hobby in the first place.

Personally, I find it really cool to just scan through the frequencies and see what's out there.

I've found pagers, police car radio, taxis, satellites, and airplanes.

It's pretty neat that a $10 usb stick can allow someone to listen in on such a variety of things.

You ever look up into the sky and see an airplane and wonder about it?

Here in the future, the answer's no further than your smartphone! Where it's going, where it's from, what it's carrying, freight or passenger? If you're near an airport, how busy is the airport? Which runway is that plane landing on? How many runways are there? What size airplane can that runway handle?

Just general curiosity, really.

Like similar hobbies, it's fun to play around with technical problem solving, numbers and data without the pressure of it being your real job. Often it's linked to a curiosity or experience from when you are young and you just find e tasks a relaxing escape from real world responsibilities. Also it's fun to belong to a community of people who enjoy technical talk.