One of the advantages of weapons like JDAMS is that the crew might not have to overfly the target, meaning they're less likely to get shot down. It's possible to "toss" a JDAMS at a target from a good distance away and the guidance system will take it the rest of the way. Of course, if that system fails, the bomb could miss by a wide margin.

Circular Error Probability is a measure of accuracy. It means that, for any given number of weapons aimed at a target, 50% would be expected to land with a specified distance of the target. The unclassified CEP for a JDAMS GPS guided bomb is on the order of 13 meters. If the GPS fails and it reverts to inertial guidance, the CEP is said to be about 30 meters. A laser guided bomb is actually a lot more accurate under good conditions, somewhere around 3 meters, but they're more prone to interference from things like smoke, dust, clouds, and the like. Under bad conditions, the CEP of a laser guided bomb would be worse than a JDAMS. Efforts are underway to increase JDAMS accuracy even more, as this article states.

When I worked as a crew commander on the Cobra Dane intel radar, our primary mission was to gather intel on Soviet (this was 1990) missile tests. Based on the data we gathered, the analysts could determine the CEP of the missiles based on things like the stability of the warheads.

The radical increase in accuracy has allowed us to go to smaller bombs than ever before. In the early days of ICBMs, the CEP was on the order of miles, so they carried multimegaton warheads to take out targets. As the CEP got better, smaller warheads could destroy the same target, so a given missile could carry more of them or you could use smaller missiles. According to this source, the CEP for the 1st Soviet ICBM was 2 nautical miles. By comparison, the CEP for their new SS-27 is about 350 meters.

The same applied to cruise missiles. The early air launched cruise missiles had CEPs of several hundred feet, so many of them were fitted with small nuclear warheads. As the accuracy improved, people realized you could take out a lot of targets with conventional explosives. Eventually, all of the nuclear warheads were replaced by conventional explosives because the CEP was just a few feet.

The same thing is happening to aircraft munitions. In WWII, we had to send out 1,000 B-17s, each dropping 9 bombs, to have a high probability of putting 2 of the bombs on a specific target. Today, a single F-15E can carry 5 or 6 JDAMS bombs to attack different targets, each with a high probability of success. Think of that, 1,000 bombers (each with a crew of 9 or 10 men) escorted by hundreds of fighters to attack a single target verses a single plane (with a 2-person crew) to attack up to 6 targets. Not only is that a lot more effective, it greatly reduces the collateral damage. The CEP for a B-17 was well over 2,000 feet.

As the accuracy continues to improve, they're working to develop smaller bombs to go after specific targets with less collateral damage, such as the Small Diameter Bomb/Small Smart Bomb project. It also allows an individual plane to carry a lot more bombs on a given mission, enabling the attack of a lot more targets. This is especially useful for stealth aircraft like the B-2, F-22, F-35, and the UCAV that must carry the ordance internally to keep the radar cross section low. Instead of only being able to carry 2-4 2000 pound bombs, a UCAV (unmanned combat aerial vehicle - currently under development) might be able to carry over a dozen bombs, each with a CEP of 5-8 meters, perhaps less.