There are many unscrupulous employers out there who would happily take advantage of their workforce, take advantage of goodwill shown by individual employees, who will take shortcuts with safety, or ignore employment law if they can (regardless of what anyone may think of specific laws, they are nonetheless laws). Employers who will discriminate, or use spurious reasons to dismiss staff without following due process. Some of this may be intentional, often it may be down to bad training or insufficient knowledge of employment law on the part of HR or management. Unions play an important role in all these areas (and can often be of help in making employers aware of their obligations thus avoiding punitive fines or other measures). Unions will also advise employees when they do not have a case, when pursing something would be fruitless, when an employer is in the right. It is not all about striking and pay rises.
I am sure there are good and bad union leaders, in the same way there are good and bad employees, and good and bad managers. Unfortunately if you are a good employee working under bad management, leaving your job is not always a feasible option. Being able to call on the expertise of someone who can help improve the situation you are having to deal with can help redress the balance.