Traditionally semi-basements were designed in larger houses where staff was housed. A semi-basement usually contained kitchens and domestic offices. The advantage over a basement is that a semi-basement is lighter as it can have windows, albeit interior windows that are often too high to enjoy a view. Historically this was an advantage as the servants, who traditionally inhabited such a floor, would not have the opportunity to waste time by looking out of the window.
The feature also has the aesthetic value of raising the ground floor, containing the building's reception rooms higher from the ground in order that they could enjoy better views, and be more free from the damp problems which always arose before the days of modern technology.