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. Used commonly with large volume dust extractors that use large diameter hoses going most of the distance, metal pipework is common.
For those wishing to know the internal and external dimensions for connecting to existing machines or other setups – external 100mm on one side / to external 150mm on the other.