Chicks absorb the egg yolk just before they hatch, and that provides enough food & water for their first few days of life.
Chicks that hatch under a hen will spend those days mostly underneath her, but they will keep popping out to eat little bits of food and try drinking. By the time they use up the yolk, they already know how to eat, they have already eaten some food, and they know that they won't eat at night so they need to fill up before bed.
For chicks that are shipped through the mail, those first days are spent in a box with no food and no proper day/night cycle. By the time the chicks arrive, they badly need to eat and drink, but they don't yet know how. They are also tired from being jostled around in the mail, and need to sleep. And of course they are cold as well.
Keeping a light on during the first night or two, as well as during the day, will let chicks alternate eating & napping all night as well as all day, which can be important for helping them recover from the stress of being shipped. (Some chicks will be fine anyway, but for some others that is very important-- and of course you can't really tell which ones need it until after it's too late.)
Other reasons for using a heat lamp with shipped chicks (light & heat from the same source) is that they can be getting warm while they are eating and drinking (not possible with a brooder plate, where they must choose warmth vs. food). Also, the light attracts the chicks toward it. They have to learn to go to a heat source that does not make light and does not cluck to them (a mother hen clucks rather than producing light.)
Once the chicks have had a few days to get over being shipped, and they have learned to eat and drink, and they have gotten several good meals inside them: darkness at night is not a problem. As you have read, it can even have benefits from that point on.
Red lamps are better than white lamps, as regards chicks pecking each other. That is why they are so popular.
White lamps in the daytime are good for helping the chicks tell day from night. So regardless of whether you are providing the heat from a red bulb, or a heat emitter, or a brooder plate, it is still good to provide normal white light in the daytime (but that light does not need to provide any heat.) The daytime light can be sunshine, or any kind of lighting that works for letting people see too.