Things unions are responsible for:
40 hour work week.
Child labor laws
Safe working conditions
At least one day off a week for hourly employees.
Unions are generally very important overall to keep companies from being dishonest. When someone with a 'bright idea' figures out you can fire everyone - and have a temp company come in with no benefits - while the temp company hires all the fired people - who take a pay cut - etc. These things happen - today. They are the result of pressure from stock holders and management to 'save every nickle' and spending *all their energy on the job* finding ways to shave money from the worker if possible - which isn't 'evil' or 'wrong' in itself - but becomes so when the individual worker has no power to fight against tactics that harm them. This is why unions are useful - they give vastly more power to the 'worker' than they otherwise would have.
Despite the rhetoric - it is very hard and expensive to sue in this country - cases that make the news are always dramatized - but the truth is you aren't going to sue your employer because almost all states (even the super liberal ones) make it so the employer can fire you for almost any reason - and it's really up to you to decide to work there or not. Yes you don't have to put up with harassment - but it's still legal in all 50 states for your boss to tell you how to cut your hair and fire you if you don't comply with no recourse (assuming you aren't bald due to a medical condition - which is about the only 'out'). Unions tend to stop petty bullshit like this and enforce policy and rules based on contract law - which is strong law. Companies benefit from this because they can negotiate with a single entity - and so they can win concessions that would be impossible to enforce on large working populations otherwise.
The bad side of unions - is that the same contract that protects you from a power monger manager, also protects the idiot screw up - and processes that are put in place to protect workers from over zealous or malicious management can also drag out firing someone who is very bad at the job. This is where a large amount of complaining happens - because it's usually easier for a company to just shuffle someone into a job they can deal with - than to fire them - and everyone grumbles (the company for how hard it is to fire someone, and the workers for having to make up slack for 'screw up bob'). Sometimes people abuse this system - and it breeds resentment. Other large problems with unions - is that power corrupts - and you are just as likely to have a power hungry union boss, that uses the power for bad ends, as you are to have a bad manager that is petty and a pain in the ass to work for.
People forget that - and thus union leadership can become a cesspool - so you need to be careful - however if the union has a good method of electing leaders, and is clear and transparent to the membership - there really isn't any downside to joining a union.