What is the purpose of floor concrete in this upstairs wooden condo?
That type of subflooring, often built with "gyp-crete", a self-leveling gypsum slurry that's poured into place, is used as a sound-deadening measure. Its primary function is to dampen foot traffic and other sounds of living for the benefit of residents below. It's a common practice in budget-built hotels, apartments, etc. where pre-cast or other concrete floors aren't used.