Langstroth was the first person to make practical use of Francois Huber's earlier discovery that there was a specific spatial measurement between the wax combs, later called the bee space, which bees do not block with wax, but keep as a free passage. The bee space has been incorporated in movable-frame hive designs since then. more details
Bees will use wax and/or propolis to close up any gap beyond a comb that is smaller than 6mm, attaching the comb to the adjacent surface.
Gaps wider than 8mm will be utilised by the bees for building extra comb, mostly for drone cells, but also for honey.
Maintaining the bee space (6-8mm) between internal hive components makes it easier for the beekeeper when removing hive components as part of hive inspection. It keeps the work environment cleaner and tidier and reduces the amount of bees getting squashed during hive inspection.