Humidifiers can come in all shapes and sizes, bread box sized to furniture sized consoles. But they all do the same thing and that's put moisture into the air, which a water fountain does naturally as it recycles the water.
Just like a humidifier, a fountain adds moisture to a dry room, while it pulls dust particles out of the air. Also just like humidifiers, fountains help revitalize your indoor plants and reduce static electricity buildup. And just a small amount of moisture in the air will alleviate dry skin and reduce allergies caused by dry air. But unlike any humidifier I've ever seen, water fountains are a lot nicer to look at and listen to.
You may have to add or change the water to your fountain (just like a humidifier), but unlike a humidifier you don't have to change any filters. Some people recommend putting bleach into the water for a water fountain, which I think is a nice cheap idea. But if you have an animal in the house you might not want to do that, because if their water bowl runs dry they might use the fountain for water. Which leads me to another benefit for a water fountain (Our lil' friends can't get water from a humidifier!).
It just seems logical with all the things I have to do in my life and I'm going to spend about the same amount of money on something, I would rather enjoy my extra time than make it more difficult.