Because a galvo laser can move so quickly, the ability to have more power is usually a nice thing to have as we can go faster for the given job and still deliver the same engrave. Galvos go over the same area over and over again to produce the job or cut desired, so having the increase in power shortens the timeframe required to complete the task.
Whether or not the wattage will perform to your liking, largely depends on your application and desired cycle times. The most common reason for increasing the wattage is to perform deep engraves in less time.