A goalkeeper is the last line of defense for any team. While goals are scored on entire defenses, a great goalie can bail out even a subpar defense a lot of times. Very few teams are successful without having a great goalie on their side.
This position is so important that it is the only one in soccer left that requires a specialist. They work with their own dedicated coaching staff, and entire teams are trying to scheme ways to figure out how to get the best of an opposing goalkeeper.