There is a huge difference between a UK "dispatcher" and a US "dispatcher". There is much confusion because of the same name for very different functions.
In the UK, an aircraft dispatcher is someone who deals with the aircraft at the gate, ensuring and supervising the loading and unloading operations at the gate. They are trained by the air carrier for this but not certified by the UK CAA. In the US this function is generally called a "ramp agent".
In the US an "aircraft dispatcher" is certified by the FAA as noted above to the equivalent knowledge of an ATP,and trained by the air carrier to do two specific functions. Preflight planning and inflight monitoring. In addition, in most US operations, particularly domestic and flag part 121 scheduled flights, the aircraft dispatcher has joint responsibility with the captain. They can and sometimes do, delay, divert and even cancel flights when necessary, sometimes even if the captain disagrees. Of course the captain can also delay, divert and cancel a flight as they feel it is necessary. The captain and the dispatcher are legally required to agree on a safe course of action. US dispatchers are also responsible to ensure that the flight crew receives all necessary safety information while the flight is enroute, such as weather, ATC issues, airport problems, security issues, etc. They can also even declare an emergency when they feel it is necessary. When 9/11 happened many US dispcthers got their flights on the ground even before ATC could. This system also requires a separate inflight communication system to ensure that a flight can be monitored by the aircraft dispatcher from beginning to end. It is a much safer system than is required by JAR-OPS or EASA. The only thing that resembles a US dispatcher in the UK is a flight planner function, but they are not certified nor responsible for any flight monitoring. Flights are often sent out and no one at the air carrier is watching them to alert the crew for weather ahead or ATC problems, airport problems, or whatever. And ATC is simply not going to be able to provide this kind of service. As a result, there have been a number of incidents and accidents in Europe where aircraft ran into severe weather, or continued flight in an unsafe condition, which was simply unnecessary. Some of these resulted in the destruction of the aircraft, while others resulted in fuel emergencies. This type of thing is very rare in the US.