Aside from the fact that I personally love ice cream, I also think it works really well as an onboard snack/dessert. Doesn't require any prep, simple to serve (though often too hard) and a general crowd pleaser as c-w-s says. I think the American sundaes in J/F work well too.
I guess the only thing is does it cost a lot to keep frozen, but I'm guessing not given many airlines do it.