Step down from kitchen to family room

[Guka Kagu]

I noticed in 1980s and 1990s home, there's a step down from one room to another. Living room, dining room, kitchen are all at the same level, but family room has a step down. Just one step and its not really sunken in. Why? I don't understand the concept. Does it have to do anything with the structure, fireplace at some homes, or something else? Basement ceiling is at the same height everywhere