Concrete?

Last night I took out the air conditioning vent in the master bathroom, so I could take a look at what's under the '70s ceramic tile. To my shock - and disappointment - there wasn't a layer of original tile anywhere to be seen. Only a 3-inch-thick layer of depressing concrete. Under the concrete is a metal grate-looking thing, and under that are the original wood floorboards. Why would someone pour a layer of concrete down in the bathroom over a crawl space? Is this normal?