Oil Additive: Why and Why not?

Chemical (synthetic or mineral based) additives engineered into a motor oil by highly skilled chemists, scientists and lubrication engineers. But why we or some still go for the extra miles to get ADDITIVES?

The PRO: The same reason we take mineral/ vitamin suppliments daily.
The CON: Will this extra elements react adversely to the existing chemicals with the engine oil? There are many questions more to the answers.

The Additives act as:
1. friction modifier
2. polarised effects
3. .. lists go on