With a blastgate at each machine, you can control the air flow from the DC unit as if you only had one machine hooked up, providing the most efficient use of the air suction. By opening and closing each blastgate, as needed, you will get the maximum amount of air flow at all times. For a more "durable" blastgate, there is a metal version available at a premium. But I have found the plastic gates run into problems with use. They jam with debris and cannot be taken apart to clean. The spot welds, which hold them together, will lossen with use, then the sliding gate, which is a friction function, will not operate properly. The metal gate uses a screw to kept the gate open.
Overall, it is a needed product that allows for maximum use of a dust collection system, with a blastgate installed at each branch to control the air flow to the machine in use. BUT, do not invest in the plastic version, go with the metal blastgate.