A "square" is not really a strange unit (at least I don't think so). It is an area - the area of a square 10ft by 10ft - which is something you can reasonably imagine. It has been in common usage for domestic dwellings for many decades - and while square metres is becoming more prevalent, the old "square" is still used.
What is funny as hell, though, is when you have someone preparing a listing for a 15 square house and they "correct" the measurement ... to 15 square metres.