To me campaigning for PSL seems more important than actually voting for PSL. Neither are deluded into thinking they can win the election. But the Greens have a 5% goal where they get can get national campaign funding. I'm not sure what the goal is for the PSL other than publicity.
Probably the same goal as any ML party: Spread revolutionary ideas to wider audiences, expose contradictions in the bourgeois political system, develop organizational capacity and connect with and educate the masses. The vote number itself can also be helpful as a structure test for self-analysis. Where was PSL successful, what tactics worked, how did that contribute to cadre growing, larger protests, etc.