Introduction
Sodium benzoate is mainly used as a preservative, and is commonly known as the additive E211.
It is most effective in acidic
conditions (ideally less than pH 3.6) and prevents the growth of many types
of microbes including yeast, bacteria and fungi by interfering with their
ways of making energy. It does this by producing a small amount of
benzoic
acid, which is toxic to the microbes. 
It is put into many food types because of the length of time between when the food is first made or packaged and the time when it makes it onto your plate!
Taken from: http://www.smokybayfish.com/html/health.html
Lots of foods can change dramatically over just a few hours. They can spoil, change colour and flavour, and lose many necessary nutrients. These changes are what companies try to delay by adding sodium benzoate to their products.