Memes

The party is necessary to actually overthrow the government. Voting for said party helps show that the present system is rigged, and helps get the word out about their platform. See Lady Izdihar's diagram on the Leninist theory of revolution:

In short, PSL is a revolutionary party that engages in electoralism not for belief in winning, but as a part of revolutionary strategy.

Should we Participate in Bourgeois Parliaments? - V. I. Lenin

Even if only a fairly large minority of the industrial workers, and not “millions” and “legions”, follow the lead of the Catholic clergy—and a similar minority of rural workers follow the landowners and kulaks (Grossbauern)—it undoubtedly signifies that parliamentarianism in Germany has not yet politically outlived itself, that participation in parliamentary elections and in the struggle on the parliamentary rostrum is obligatory on the party of the revolutionary proletariat specifically for the purpose of educating the backward strata of its own class, and for the purpose of awakening and enlightening the undeveloped, downtrodden and ignorant rural masses. 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.

Lenin's argument was that even if an electoral system is deeply flawed, participation is necessary in order to reach people who are invested in the system. In addition to spreading the message, it also provides a way to assess the strength and popularity of a platform, and it can serve as a means of testing and weeding out prospective leaders who might be opportunists.