I can't speak for the OP, but I have customers I still support, because they supported me many years ago when I was a teenager taking my first steps into industry.

Does it make me money? Barely a cent. But I can spare a hour or two a year for the guy who gave me a leg up and trusted a teenager who probably shouldn't have been trusted. And I like the feeling of having something I worked on still going strong 20+ years later, when so much of my later work has been thrown away by the endless corporate rewrite treadmill.

They've always paid me per hour. The fewer hours, the better for me: just like the sibling post, I'm also not in it for the money. I care for both the customer and the project, and I'm happy that we've found a way to get the development going again with really minimal effort from my side.