Wood and many wood-like materials expand and contract with changes in temperature and humidity. If a wooden floor is laid tightly edge-to-edge with no space to move, the planks can press against each other and the walls as they expand.
Since the floor has nowhere to go, it starts to buckle or lift in places, creating ridges, gaps or even damage. It can also put stress on skirting and door frames.
By planning small expansion gaps at the edges – usually hidden under skirting boards or trims – you give the floor a bit of breathing room. It can swell slightly in humid months and shrink back in drier periods without causing visible problems.
Good installers know this, but as a homeowner, understanding it helps you check that the job is being done properly.