Actually the only way for a "my IP" application to make some money is be indexed as first result in Google I guess. At least I never remember the domain of one of that 200 trivial apps and search on google. So makes a lot of sense they showing you the result if you search for "ip" or "my ip".
I bet some of them find alternative ways to get visitors and that a decent number of people use the bookmark feature. Now, how valuable a visitor is to a page like that is another matter.
