A single step reducer allows you to create some consistency within your dust collection system design. Using larger diameter as the main pipeline back to the machine, you can reduce to a smaller hose diameter off of the main line to power tools or stationery machines that have smaller dust ports.
For those wishing to know the internal and external dimensions for connecting to existing machines or other setups – internal 119mm / external 124mm one side to internal 93mm / external 100mm.