Does food need a halal certification?

I have seen symbols on food in supermarkets and restaurants marking the food as halal. I assume that these symbols come from authoritative Muslim food supervision companies. I have also seen Muslims who are careful about halal eat in other restaurants and eat unmarked food after checking all of the ingredients. I also thought that all food was supposed to be considered halal until proven haram. So, does food need to have a certification in order to be considered safely halal? If not, why do the certifications exist?