What is the purpose of multiple layers in Printed circuit board? What are they required for?

One way of thinking about the tracks on PCBs is to compare them with highways across large busy cities.such as interstate highways stacked 5 levels high, towering hundreds of feet into the air!

Printed circuit board traces are somewhat like the super-highways. Each lane typically takes traffic in only one direction (ok, a lot of pcb traces can take traffic in two directions, but otherwise, this is a decent analogy). Stoplights only slow the traffic down. If you want to get from one part of a large city to another part many miles away, you may have to use overpasses or underpasses to avoid traffic on other highways that your’s intersects.
On a PCB, a feature called a via connects traces from one layer to another layer. Like traffic in small towns, the tracks on the top and bottom of the board where the cities are located often have local roads traveling in many directions - taking curves and roundabouts as required. But once they reach a via connecting them to another layer (in this analogy, think of a via as an underpass, an overpass, or simply a manhole!), the lanes on the inner layers often act as interstate highways, distributing traffic simply from one edge of the board to the opposite edge of the board. Unlike tracks on the top or bottom where the cities are located, the inner layers are usually assigned a direction. They are either fast lanes from north to south, or fast lanes from east to west. On military boards for missiles they are often circular rather than rectangular, in expanding circles. In interstate highways, these are the highways with 3 digits starting with “2”. The higher the first digit, the farther from the center of town it is located.

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