It is commonly called a "maintenance" dose...
The idea being that after a full, 2 treatment cleaning regimen this amount should actively prevent future crud from accumulating. Going to a 100% synthetic oil after the cleaning would probably have the same effect. Might cost more, too!
Of course they want you to use their product! There are other products (Neutra 131 comes to mind) available that can also be used as both a cleaner and as a preventive maintenance dose. It is considerably less expensive. I don't think it does the job as quickly as Auto-Rx can although some buy it by the gallon and use it in an almost "engine flush" mode at significantly higher concentrations. Lots of discussion on the Bob-is-the-oil-guy Board. It can be added to gas, also.
It all depends upon whether the idea of cleaning up the internals of your engine and keeping it clean are attractive to you. Some motors are so gunked up that more 2 oz. per quart treatment applications are necessary. If I were to see really dirty, dark oil & lots of junk still coming out of the pan & filter folds after two applications I for one would not hesitate to keep going until things cleaned up. Most engines are usually not this far gone.
'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435