Stems swell at their joints.

The primary reason is to be strong enough for the leaves and flowers that emerge. The second reason is to transport water and nutrients easier throughout the plant. The thicker the stem and branches the easier the transportation with water and nutrients will be. Cannabis plants are very strong and flexible through a combination of the strong fiber making up the stems and branches, the process of osmosis that keeps cells firm, and the hollow space within stems and branches for flexibility under duress.