My daughter was one of two captains for her middle school team last spring. They weren't given any real softball duties, like leading drills, or telling people how many outs there were. Their main job was to answer teammates' questions, like ''Do we wear white jerseys or blue today?" or "Are we wearing our spirit wear on Friday'' or ''I forgot if it's a home game or an away game Tuesday'' or ''Where are we supposed to meet for the pep rally?" My DD was constantly getting texts like that.
Also, the captains took the lineup card to the umpire (accompanying the head coach). And they would decide what players would wear to school the day of games (choices - spirit wear, jersey, dress up nice, or whatever you want).