What is the origin of the idiom "in on the ground floor" and when to use it?

The idiom "in on the ground floor" originated in the world of real estate and construction. It originally referred to being involved in a project or venture from its initial stages, particularly when a building was being constructed, and had a literal connotation of starting from the first floor of a building, which is the ground floor. Over time, it evolved into a figurative expression, denoting the opportunity to participate in or witness the early development of a venture, idea, or trend, with the implication that those involved from the outset have a potential advantage.