All those medieval houses one seas with the upper floor overhanging the lower floor? Like this.
To gain space.
The structural advantage of the jettied wall counteracting forces in the joists or tying a stone wall together
To shelter the lower walls of the house from the weather.
Uses shorter timbers, a benefit due to timber shortages and difficult handling of long timbers especially in city streets.
As a status symbol or "...symbol of wealth and status.
I would imagine there is a little bit of all in that.