This "por" here means "because of" (the cause). The sentence with "para" which would also be correct but would have a slightly different meaning, that the party would benefit from their "effort". If I give you other examples you will see it a bit more clearly:
"Lo hago por ella, no por ti." (I am doing it for her, not for you.)--> here, she is the cause of you doing something (because you care for her)
"Che Guevara luchó mucho por la revolución cubana" (Che Guevara fought a lot for Cuban revolution)--> we see Cuban revolution here as the cause of Che's fighting.)
There is a very thin line here with the use of por and para, but using por adds this specific nuance.
I hope this helps.
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