All industrial action is not to benefit those on the breadline. If only those on the breadline were allowed to strike, there would be a race to the bottom (affecting everyone), and only those on minimum wage would be in unions.
They exist to protect all members, whatever their wages or salary, from exploitation and to fight against job losses as well as diminishing pay and conditions.
In large companies and organisations they make bargaining between workers and the level of management that decides pay awards more sensible. If everyone had to negotiate their own pay deal it would make life more difficult (and that applies to all grades of staff in large organisations - obviously individual negotiations are more common in small companies).
They liaise with government about employment legislation, and provide workers with legal representation when necessary.
Unless we have a system under which everyone earns the same, there will always be some who earn more than others. Are those who say this strike shouldn't happen because train drivers are relatively well paid saying that everyone should earn the same? If so, how would they persuade people to do jobs that others don't want to do, or to work in dangerous conditions, or to spend years training to gain skills that others don't have?