Pre-grant submissions can be used to challenge a competitor's patent application and prevent the patent from being granted or reduce its scope.
The America Invents Act (AIA) makes it easier to limit the scope of a competitor's patent before that patent issues. Prior to the AIA, one often had few options other than waiting until a competitor's patent issued and then challenging it or filling an interfering patent and provoking an interference.
Now potential targets of a pending patent application have a better option. They can submit published documents relevant to the examination of an application along with a concise explanation of the relevance. Thus the examiner may consider prior art that would likely not come up otherwise. Such submissions could certainly be pivotal in the scope of the patent that results from the examination.
Submissions must be made before the first office action or within six months of publication, so one must monitor a competitors activity in order to make pre-grant submissions. However, a timely pre-grant submission could easily save millions.