In my own words: A labour union is a group of workers who have banded together for better pay, benefits, and conditions at their workplace.
If you like those things, you should organize a union with your coworkers. If you already have those things, you should organize a union to keep them.
... if only it was that easy! Why don't we "just organize"?
Most people support unions. But some, especially younger workers, don't even know what unions are. (I didn't know what they were until my mid-20s...) If there are less people in unions, then less people know someone in a union, let alone what unions actually do for workers.
Another issue is time and effort. Employers are good at squeezing hours and energy out of workers. To maximize their profits, they minimize employee wages. They also minimize downtime: breaks, rest, even meals. Employers hate unions because it means they would have less control over their workers. In turn, that means less control over their profits - how much they make off of your work.
Employers usually earn a lot more than their workers. They use that money, time, and control to prevent unions - union busting. Your boss might intimidate you, or create divisions between you and your coworkers.
A coworker might mysteriously get a raise or benefits, while others get nothing. Or an entire staff gets punished for something one worker did. Reprimanded for speaking another language during a break.
It's pretty damn hard to organize. Workers that need a union the most are often ones with low wages. The worst part of their jobs is they don't earn enough for savings. There's nothing cheaper in the store - they just go without. Low wage jobs also usually mean low job security. The system is rigged against workers. Even if workers win a union vote, employers can stall the first contract and make their workers' lives awful in the meantime.
But unions have to start somewhere. The 8-hour workday was once 10, and even longer before that. It took time and effort from workers to fight for better. Things can change, and they should.
The workplace is where most of us are. Us being people who have to work for a living. We give the best hours of the best days of the best years of our lives to employers, as economist Richard Wolff has said. It's worth improving the workplace for yourself and your coworkers.