Depends how old your house is. In older houses, plastering wasn't done right down to the floor because the best floors were made with wooden planks. These were not laid directly on the earth below for fear of damp and rot, which meant that the wooden floors were 'suspended' with a gap below them for air to circulate and clear away dampness from stale air; hence air-bricks in the walls below floor-level. The gap left at the bottom of the plastering was to anticipate the up-and-down flexing of the ends of the planks as weight was put onthem inside the room which would crumble plastering they met. Protection of the plastering from cleaning of the floor was another reason. Modern houses usually have solid floors which don't flex, and walls can be plastered right down to the level of the floor; skirting boards are then decorative, but they do protect plastering against damage against knocks from feet and furniture.