Cars and trucks have had bumpers for about century. They were originally meant to take the impact in a collision and be easily and inexpensively replaceable. It's likely the most common reason it would need to be replace is if you were in a collision that damaged it. Sometimes they can be repaired with there's just a small crack, but that can cost as much as a brand new one. Another common reason to replace the bumper would be if you're restoring a vehicle and the original is either rusty and corroded, or just doesn't have that same luster it did when It was new and it's not worth refurbishing it.
Up until around the 1990s, most cars and trucks had independent, bolt-on bumpers that were either chrome and shiny or painted either a black or body color. The chrome ones were the earliest and were used all the up into the 1970s. Before then, the bumpers were made from steel, very durable, and very heavy. They didn't really "tie-in" to the rest of the vehicle and instead of making them an eyesore, the designers just chromed them like all the trim so it looked fancy and more expensive. Starting in the 1990s is really when most passenger cars started using what are called bumper covers. These are large pieces of plastic that install over the true, metal bumper so the car can have a more streamlined design. In the event of a collision, the plastic would virtually disintegrate and the real metal bumper underneath would absorb most of the impact. Even in the last few years there's been many trucks designing them this way as well.
If yours needs to be replaced, look no further. Car Parts Discount has a selection of bumper cover parts for a variety of vehicle at the lowest possible prices.