What is used to make weapons accurate?

Most weapons in the past used skill and timing to hit the target, and sometimes, in the case of carpet bombing there was no specific target and they simply blew anything in their path into pieces. Modern weapons use laser-guidance or GPS systems to track down the target. Some lesser used systems have a digital picture of the target and the surrounding area, once an eye on the missile sees that it is in the correct area, it searches for a matching image and hits that target. The problem with that system is that changing certain things in the area can effect the accuracy of the missile. Why is accuracy important in the military?-----What things previously thought to be accurate aren't?