If you work full time (~2080/yr), then $700 amounts to $0.33 (33¢) per hour, or about $27 per biweekly paycheck. Any union worth their salt will certainly get wages increased by more than that. Unfortunately, for a lot of people at or near poverty, that $27 makes a huge difference.

Same thing with fighting anti-union activity by your employer - yeah, it's federally protected and you'll likely win in court and get back pay, but few can afford to not get paid for the intervening months while it works its way through the system.

Unions aren't even that expensive, here they cost about €4-20 a month, depending on your net disposable income and whether you study or not. At worst, that's €240 a year.

Take into account that with a union I might get a 5% raise instead of 3% on €30k, and it more than pays for itself (an extra earn of €600, minus union fees that still yields an extra earn of €360).