He is thinking about making a small intimate lounge on the ground floor under the stairs. It is a nice little cosy place and the light coming from above creates that special atmosphere. There will be no electric lighting under the staircase.
You have to be careful with those gaps, especially if children come into the equation. The handrail should be high enough in order to avoid accidents.
Also look at the way the balustrade is executed. Those simple curves and natural wood – what can better than that? Also the colour scheme – polished wood and cream-white matte paint – makes it simple and organic.