HR is paid by management to manage -- that is, assure that the company can get the most out of for the least cost -- the "human resources" of the company.
Unions are paid by their members to assure that those members can get the most out of their employers for the least cost.
From the employees point of view, it is quite literally true that HR is not there for you whereas unions are (or, more accurately, the union you are a member of, if any, is) there for you.
> We also had an option to take out of our paycheck a monthly fee for legal advice and retainers. I got that option just cause I thought it was a nice security I really don't know why I would need it. It turns out these retainers cannot help you legally against your boss or company btw.
Yeah, employer-arranged group legal services plans never cover any scenario where you are against your employer; they are something the company arranges so that employers can be less distracted from serving the company's interest by outside legal matters.