Humans make mistakes, usage of technology to overcome the mistakes is the third umpire. While watching on television we easily conclude it isn't a wicket. But, the third umpire cannot do the same as we do. He needs to look it upon various dimensions, because the same will term to be controversial. Some even change the result of the match.
Managing a game requires so many necessary things or equipment. Since the cricket field is large in size, other technologies with sufficient number of cameras have to be resorted. A third umpire also has to take care of various things. In this case, there may be some time lag but it is in the interest of accuracy which is tolerable for cricket.