I've been looking into the K2.css file (in components/com_k2/css/) and I've noticed the stylesheet file is really very inefficient. It contains 294 empty style declarations like, mare than 1/3 of the styles:

span.userItemEditLink {}
span.userItemEditLink a {}
span.userItemEditLink a:hover {}

Why are there so much empty and useless lines of code?

Because its purpose is to provide a scaffold for users who want to further enhance the styling.

You can always override the stylesheet and remove the blocks you do not wish to use.