I'd had to ask the first time I saw one, too. Though there was something of an inkling (I'm something of a flight nut, and the property was close to the local airport), I wasn't 100% sure, so I asked. Turns out the one I saw also included a provision that no structure would be erected on the subject property over a height of X-number of feet, which would have obstructed the runway approach.

Always learning is a great thing, isn't it? :)