If war or political upheaval in a major oil producing country restricts the supplies of oil, this can have a major effect on the world economy
Countries that cannot produce their own oil are left at the whims of oil producing countries who are able to set the prices of oil
If a poor country cannot buy all the oil it needs, there may areas deprived of electricity
Oil producing countries can, in effect, hold other countries to ransom by cutting off the oil supply if there is a political disagreement
However, oil industry also produces many jobs