It probably supports wifi too. But if the power in the house goes out, so does the Internet. By going cellular they avoid that problem, although batteries can still run out.

I'm not convinced this is what we need. I suppose if you littered your house with these sensors and could see a "house dashboard" to check the house's statuses on a web page any time of the day, it would be nice. But this really won't solve a busted pipe in the basement.