If for no other reason, then because the election gives an opportunity to advocate for a socialist platform and ideals. According to Lenin, socialists have a responsibility to participate in bourgeois elections until the people have given up on them and stopped paying attention to them.

Whilst you lack the strength to do away with bourgeois parliaments and every other type of reactionary institution, you must work within them because it is there that you will still find workers who are duped by the priests and stultified by the conditions of rural life; otherwise you risk turning into nothing but windbags.

Our culture considers how you vote to be the defining aspect of your political character. Voting for a specific candidate makes people feel more inclined to defend that candidate's actions. Not voting promotes disengagement from politics altogether. Moreover, a party like PSL can use the attention it gets from the election to promote itself which it can then use to organize in other ways beyond elections.