To flag things for removal that the powers that be, aka you, don't want on HN, as defined by the Guidelines. What I want on HN and what you want on HN aren't the same, so users like me have to go by the Guidelines on what is and isn't flaggable. For example, some types of submissions I utterly despise but don't violate the guidelines include "product launches that lie about being the first/only way to do something when this tool already exists", "obvious bullshit like a ten year old curing cancer", "this month's revolutionary new battery technology, ignore the fact it has so little real application we aren't even patenting it", and the classic "here is a five page article examining a question whose answer is not only incredibly obvious but can be answered in a single sentence".

I'd be overjoyed if you told us you wanted us to flag that stuff.