a union represents the workers of a company, so if they’re being mistreated they can push back as a collective rather than just one person. say your store routinely misses breaks, or there’s some stupid rule about going to the bathroom only on your breaks, if one person comes forward to complain they’re a lot easier to ignore vs a whole group, + a lot of the time they’ll come with demands like higher pay as the big one, the union and the company make a contract with those demands and if the company/manager breaks the contract then the union can sue