The human being is so complex that one can express her emotion and attitude by using hundreds of kinds of facial expressions.
They can show you what they feel about you by moving their eyebrows, by smiling... Let us say for example, you are telling jokes at a party(I'm not sure if it's that good to use "let us", ... but I have no other idea). You can infer this from people's faces that you are not very good at making people laugh(whether your joke was a good one or not.). Therefore, you can adjust your behaviors properly before you make fun(/a fool out) of yourself. However, if you use e-mail or instant messages, you simply see texts and it is very hard to know if the others are willing to communicate with you.