I actually remember where I stole this qs from: ABC Australia's "VoteCompass" quiz

Unions are beneficial in most cases to the worker, I believe that the average worker should have some power in the decisions that affect them. Without the union, some of the worst unsafe conditions would persist. But, not all unions are created equal. I think the concept of the "one big union" is a bad one.
Unions should be for various job classifications, and should have requirements to become a member.

Well, to be more specific, they have too much power, but I think their intentions are good. At least the ones at the mines, they're always bitching about how much worse it is there than at other places. Then again, the intentions are good, but the group as a whole is too much of a selfish asshole to work. It’s because the mine owners fund them, and the mine owners are all a bunch of fucks that only care about profit and don't give a shit about the their workers.
Union motherfuckers better concentrate on raising the wage for their lazy ass members before they start bitching at someone about running a profitable business.

They don't have enough power.

What is trade unions

^Unfortunately won't be able to explain in my 3 min coffee break but will make a proper comment tonight :)

So a trade union is an organisation of workers who have come together to protect their common interest - ie. maintain or improve their conditions of work. This can be done by attaining better wages, improving safety standards, getting employment benefits (eg. healthcare, retirement), protecting workers against horrible bosses, etc. A union often organises workers in a particular industry (eg. a steelworkers' union, an auto workers' union, a retail workers' union).

A single worker has no power compared to a corporation. But a large group of workers? They vastly increase their bargaining power by banding together. They can negotiate as a group, or withhold their collective labour (ie. strikes), or simply make the corporation sit up and pay attention.

It works off the principle that capitalists / big bosses need the workers, but the workers don't need the capitalists / big bosses. If all workers lay down their tools tomorrow, the world would come to a grinding halt. If the big bosses all called in sick? Life would go on.

Some things we take for granted that were achieved through the efforts of workers' unions:
- Weekends
- 8-hour workdays (many people in the Industrial Revolution were working 14 or 15 hours per day)
- Every work holiday other than July 4th and Christmas
- Meal breaks, rest breaks (before unions agitated for them, workers were required to work the whole day without a break)
- A minimum wage
- Medicare
- Social security
- Pensions
- Paid overtime, sick leave, hazard pay, maternity leave
- The right to strike
- Child labour laws
- Workplace safety standards

Unfortunately union membership has severely declined, in all western countries, since the attacks on them during the Thatcher / Reagan era. It's now basically illegal to strike in Australia. A few years ago, the US construction industry was found to have a blacklist of workers who were part of a union. Many corporations engage in link activities, which have sometimes been violent.

There's a significant correlation between the fall in union membership and the increased share of income going to the top 10%, as seen on link.

Anyway the rationale behind workers' unions is basically summed up in the famous union song link:

When the union's inspiration through the workers' blood shall run,
There can be no power greater anywhere beneath the sun;
Yet what force on earth is weaker than the feeble strength of one,
But the union makes us strong.

Chorus:
Solidarity forever,
Solidarity forever,
Solidarity forever,
For the union makes us strong.

Is there aught we hold in common with the greedy parasite,
Who would lash us into serfdom and would crush us with his might?
Is there anything left to us but to organize and fight?
For the union makes us strong.

It is we who ploughed the prairies; built the cities where they trade;
Dug the mines and built the workshops, endless miles of railroad laid;
Now we stand outcast and starving midst the wonders we have made;
But the union makes us strong.

All the world that's owned by idle drones is ours and ours alone.
We have laid the wide foundations; built it skyward stone by stone.
It is ours, not to slave in, but to master and to own.
While the union makes us strong.

They have taken untold millions that they never toiled to earn,
But without our brain and muscle not a single wheel can turn.
We can break their haughty power, gain our freedom when we learn
That the union makes us strong.

In our hands is placed a power greater than their hoarded gold,
Greater than the might of armies, multiplied a thousand-fold.
We can bring to birth a new world from the ashes of the old
For the union makes us strong."

Where "they" and "their" and "idle drones" refers to capitalists / big bosses.

The above two comments were clearly pro-union so I guess I have to mention the other side too.

Milton Friedman, a famous right-libertarian economist, argued that unionisation increases wages and thus decreases the number of jobs.