This is a somewhat common thing. As Cableguy1000 mentioned above, it’s good for water conservation. But even more commonly it’s used for tight spaces. For instance I saw a Victorian home on a tv show that never had a bathroom upstairs. The owners found it inconvenient to have to go all the way down stairs to pee at night, but didn’t want to do a big renovation. The solution was to turn a small upstairs hallway closet into a quarter bath. Plenty of room for a standard toilet or sink, but not both. A set up like this was the solution.