Multinationals exist because so much of trade is worldwide, not just in your home country. They often flourish because they are able to use numerous resources and people to achieve their goals and do it more economically and efficiently than other companies. Nations that have embraced international corporations have growth rates of 5% or more per year, in comparison to 2% in Western countries and 1% to negative growth in countries who have not acknowledged free trade.
One importance of multinational corporations is to help provide additional financial resources to developing countries. Jobs then become available to citizens of these countries. This gives a ripple effect to their economy. Here is a document that the United Nations created about multinational corporations-- www.andrew.cmu.edu/course/73-371/UN_article.doc