if you want to keep the mac address of the stack same irrespective of whichever switch is the master.
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/ha_stack_manager/configuration_guide/b_hastck_3se_3850_cg/b_hastck_3se_3850_cg_chapter_010.html#concept_872363802C644BF5B10743143BE510BC