When the preposition 'for' is used with the verb 'advocate' is would mean 'for the benefit of'. Therefore, the sentence 'She advocates for foster children' is grammatically correct while 'He advocates for lower taxes' is NOT grammatically correct as lower taxes is not the beneficiary.

Please note that just because a usage has become widespread, that does not make it grammatically correct. If so, the sentence 'I seen the movie' would be deemed correct.