The way you wanted to phrase that question is, "Cool! Was there any other reason you wanted to build this beyond the technical challenge of it?" Otherwise it sounds like you're saying, "You need to get laid more, bro."

This question is also handy at hackathons. Sometimes you get an answer like, "nope", other times you get anti-government screeds, sometimes dreams of saving the world once they've scaled up.

Personally, I find it better not to ask and to just enjoy the hack.

I think a guy capable of building an automatic plane spotting system gets laid. I mean.. if not, what's wrong with the world?

The reason I asked is because I have a unique opportunity to do the same where I live but I want to know if there is any utility in it other than the build itself and the learning experience, and if so, what it is. This reminds me of the thing that takes photos of the ISS every time it flies overhead, except I get no understanding of why someone wouldn’t just use the many many high resolution photos of it from much more up close or the publicly available data. But let’s say that it turned out that every time it passes by you could use your decoder ring to get another piece of a neat message then I would get into it for my kids’ sake.

“ Is it the cool factor of being able to identify aircraft out of the sky with a camera?”

Yeah, pretty much. But for short we just call it “planespotting” in the hobby community ;)