I went to tile upstairs bathroom floor and when we pulled up old linoleum and replace subfloor that was rotted we discovered 1 1/2" of concrete poured on top of old subfloor. Why would somebody do this?
It was (and still is) common to float cement on top of the floor for installation of ceramic tile. It provides a level and firm substrate to which tile mortar (thin-set) will adhere. 1 1/2" seems a bit thick though, maybe there were irregularities with the floor which had to be compensated for. I try not to float so thick unless I have to, and now I use cementitious tile backer board when appropriate. Or sometimes I mortar the tile directly to a smooth firm plywood subfloor in new construction applications.