The theory is that table manners evolved as a way of sharing a precious commodity (food) without violence (source). That's why, for instance, the knife's blade always points to the plate when you set a table: you are signalling that you will not use it on your neighbor!
I think manners are really all about making others comfortable, and being able to show self control in the face of temptation. I understand not wanting to do those things while eating dinner, but it's always good to know how to do them, just in case you're in one of those situations where those skills are needed,