Why do the :before and :after pseudo-elements require a 'content' property?

Question

Given the following scenario, why does the :after selector require a content property to function?

Answer 1

2020 Edit

:before

Is syntax from CSS2 for better practices you should write the newest syntax from CSS3 which is

::before

with double semicolons

Full answer and differences can be found here: What is the difference between :before and ::before?