We also took a peek at devices that connect network segments together to allow some traffic to pass, but other traffic to be blocked. These packets contain all the data needed for devices to communicate across a network. Some protocols place more of this extra stuff onto the payload than others depending on what needs to happen.
Headers go on before the payload, trailers go after.
More functionality means more information required. Think of you connection to the internet as a water pipe. The thicker it is, the more can get through at once. A pipe that's thinner would have an inverse effect. If we stick with the water pipe analogy, think of not only how wide the pipe is, but how fast the water inside is traveling.