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.