Why would anyone believe what a politician says about an opponent?

I have always treated what any politician says about an opponent. Yet, time after time, we do see that negative advertising seems to work in politics. This is puzzling. If an advertisement says something negative about a competitor, I assume that this is self-serving and likely to be untrue or taken out of context. Politians are even more likely to do this, yet it seems to work. Given the scepticism with which politicians are viewed in general, this is astonishing.