The face is typically one of the first things an artist learns how to draw. Not just in the formal training, but sometimes even in childhood. A face is the number one image that a viewer can connect with. This is so much so, that people often 'make order out of chaos', wherein we will attribute a facial likeness to something that is not a face: i.e. an electrical outlet can look like a very surprised person. The reason this occurs is because the face is the primary mode of showing emotion. Body language comes in second. Emotion is one of the most basic things that a person can understand and identify with. You don't have to teach a person to cry, smile, or concentrate. We just do it, and we can recognize it in someone else immediately.
This is why an artist can take advantage of the close-up. A close-up is a camera angle where the primary subject is the character's face. It can range anywhere between including the character's shoulders and a significant area above the head, to showing one very large eye.
Using a close-up draws the reader in to a very intimate level with the character. Think of it this way: if you are actually this close to a person in real life, the
only reason you'd allowed to be that physically close to them is because you are intimately connected with them. When a person's face takes up most or all of your visual range, there is a lot of trust and vulnerability that they are allowing you to share. And if they aren't wanting that, they will retreat away from you.
Because the face is one of the first things an artist learns to draw, it is in some ways the easiest thing for them to draw. Thus, many times, an artist will cop out and pull the camera in close to the character simply because it is easier or it looks cool. Now, to be fair, a character's emotions aren't always cranked up to 11. A close-up doesn't need to be a very intimate thing, but like every other camera angle, there must be a purpose to it. For example, if someone is saying something important--even if they don't realize it's important--a close-up helps emphasize that importance. But just as pulling the camera back takes in the importance of the environment, bringing the camera close prepares the viewer for the importance of emotion.