Not that I ever cared about karma, but I can see why some people did. It plays into peoples need to be liked. Karma associates a number with how well your post is doing. Bigger number = more people liked your post. Basically quantifying how well liked your post is and then gamifying it.
If the number of atoms is a multiple of 3, then you can split it perfectly.
For example say there’s 6 atoms in a cake, and there’s 3 people that want cake. Each person gets 2 atoms which is one third of the cake.