Interpol is an interesting organization because they are so low-key. You don't often hear the word in news; in fact, I can't think (without research) of a real-world Interpol incident. I know it mainly from entertainment; books and movies are in love with the idea of an international police force, and I first ran across it in a juvenile series. Still, with the world-government theme in full force both in public consciousness -- and quietly in actual government -- I think a global police force might turn out to be a good idea in the long run.
Interpol is particularly important given the realities of the globalised world in which we live, where it is easy for crime to be a transnational business, in the same way that nearly all businesses nowadays operate transnationally. Interpol is therefore vital in its role as an international policeman and its attempt to bring the national police forces together so that they can work transnationally in unity to combat crime.
Aside from global terrorism, Interpol's highest profile activities are related to business. They deal with corruption, money-laundering, computer fraud, white-collar crime, and other crimes that cross borders in today's globalized world. They also publicize (even if they can't necessarily punish) businesses that continue to do business with countries under UN sanctions. Another very high-profile area of activity is in copyright violation via websites. It seems to me that their role will only increase as time goes on.
Interpol's main function is to act as a global policing unit (for those countries which accept its laws). Interpol helps the 190 different countries with legal business which they cannot complete on their own. Given the widespread illegal activities which exist on the Internet, policing agencies like Interpol are absolutely necessary to insure that countries are working together.
Interpol also helps us as citizens, by attempting to control international crime that can affect individuals as well as businesses. Drug trafficking immediately comes to mind. Interpol is also heavily involved in attempting to bring an end to human trafficking, as slaves are still being sold and bought and transported throughout the world.
The two above post are right on. We are living in a more globalized word and hence crimes are more sophisticated. Clever criminals are crossing borders, which make things more complicated. Hence, Interpol is very important in the fight against crime. Interpol works best when there is a sharing of information with local law enforcement. In terms of your other questions. We need Interpol, because some of the biggest crimes committed are now international. And certainly Interpol does monitor corrupt business transactions.
Since the world is becoming increasingly "globalized," in the sense that national boundaries matter less and less these days than they used to, the need for an agency such as Interpol seems strong. Many criminals (especially wealthy criminals) think that they can avoid arrest these days by fleeing from one country to another. Interpol is useful in dealing with such developments. It is probably also helpful in dealing with international terrorists and terrorism.
I assume that you are talking about Interpol, the international police agency. It is important because it helps to enforce laws against certain kinds of crime that are committed between more than one country. In other words, it can help with things like internet crime where criminals are in one country and victims in another.