The cost is virtually zero so any ad clicks would be a bonus.
Those sites are also easy to get back links to, so you can easily stuff them with links to your money sites to get some link juice flowing.
People searching for their IP are a niche unto themselves and can be sold stuff like proxy services, browser history cleaning tools, geolocation subscriptions, remote desktop tools etc. They're often not very technically savvy users.
Those sites also make a great entry point to your own network of ad-heavy web proxy sites.