It shows people that communists aren't overly-serious academics with a stick up their ass. Joking about stuff is how people communicate with one another. Communists need to communicate their ideas to people in ways people will understand and accept. People are going to actually consider the ideas of a group that they can joke along with rather than a group of stuffy, overly serious people allergic to humor.

Additionally, the far right has had this figured out for quite a while now, and they try to present "the left" as utterly humourless, because they know that this is one of the best ways to get people to never want to learn about a group. Socialism is a human-centric philosophy, we need to show people our humanity.