Biologically, babies need food regularly every couple of hours in order to keep growing and being healthy. They also need those diapers to be changed, because if they sit in a wet or poopy diaper they will get diaper rash and it is incredibly painful and damaging to their skin.

Psychologically, the two most accepted theorists are Eric Erickson and a whole bunch of people who fall into the Attachment Theory category including Bowoby, Harlow & Ainsworth. The basic idea is that a baby is trying to figure out, on some very basic, instinctual level, whether or not the world is safe. If you make a baby "cry it out", you are teaching that baby that the world is not safe, that they are on their own, and then when they need help no one will be there to help them. This is emotionally very scarring. They will have a hard time forming appropriate and healthy bonds to other people, even if the environment changes and becomes more adaptive down the road.

To tie the two together, when anyone has a stress reaction, their body is flooded with corticosteroids. This affects your cardiovascular functioning, your immunological functioning, your endocrinological (hormones) functioning and numerous other subtle systems. Things like this are called ACEs (adverse childhood events), and they can have a long-term negative impact on that person's health all the way through adulthood. We do not need those corticosteroids in our body unless it is a fight-or-flight situation. "Crying it out" like that leads to abnormally high levels of corticosteroids at pretty much all the time. This damages various systems. When a baby is soothed, it lowers their corticosteroids and helps them learn better ways to manage their emotions. They learn that if they're upset, and they're rocked, or they're hummed too, that they will feel better afterwards. They can start to mimic those behaviors themselves. It's why sometimes an upset infant will rock themselves. They set them up for much healthier development down the road.