This switch is needed when running a large boost activated secondary pump and a smaller primary pump. Most HOBBS switches activate at around 5psi. Sometimes this is too low of a trigger point when running a low boost. Depending on your engine combo you may start to run out of fuel before 5 psi.
Some cars are capable of making 600+ hp with only a couple psi and not be able to activate the second pump, resulting in a lean condition.
This switch is activated at only 1 psi