This secondary regulator allows you to keep two lines from a single cylinder at different pressures.
To do this, you attach your "normal" regulator to your cylinder, set it to the higher of the two pressures, and then split the gas line using a tee barb or an air distributor.
The line on which you want to run the higher pressure goes directly to the keg. You put this secondary regulator inline in the other tubing, setting it to a lower pressure.
This is great if you want to have a combination of beverages on tap at the same time that take different carbonation levels, such as soda (higher) and beer (lower) or a Belgian ale (higher) and a stout (lower).