One of the main reasons is that, children like the view they get when their heads are at the same height at ours. During toddler hood, kids are very small and they seem to miss most of the things that happen or exist above their level of sight.
For sure, all children feel comforted when being held. They feel soothed when they are held close to the heart.
Kids tend to be more alert or afraid of new places or strangers. They feel safer and more secure in your arms. You are the world to them and they feel that no harm can come to them as long as you are with them.
Babies demand to be carried because, until they're around 3 years old, that's the only way they can stay with you.
Children feel that no attention is paid to them. When they're at your level and close to your ears, they can easily talk to you and you will easily listen to whatever they say.