Unions are one of the workers' fundamental tools in the class struggle. This is motivation enough for their existence.

Just in case there are people out there who do not see the benefits of creating an oppression-free society I feel that I should provide some more arguments.
What it boils down to is that the relationship between a corporation and an individual employee is a very uneven one. The loss of a job is much more damaging to the employee than the loss of an employee to the corporation. Unions lessen this distortion and the resulting losses.
One area where unions have a beneficial impact is workplace safety and conditions. If workers are easily replaceable and have difficulty making themselves heard it will be more profitable for the companies to pay little attention to those issues. For the society as a whole (not to mention the workers concerned) people being worn out or injured represents a large cost.
And there are of course the wages. The result of the corporations' more powerful bargaining position is that wages are pushed low. Since the price of labour is lower than its actual cost this causes resource misallocation. The introduction of unions therefore helps the functioning of the market economy.

It is also worth considering what sakico mentions above: a company shutting down rather than paying the wages demanded by the union. According to sakico this shows that unions are damaging, and that union busting should therefore not be discouraged.
We can distinguish between two cases:

Paying the wages demanded by the union would make the enterprise unprofitable
To quote sakico: "Good riddance". If the company can only make profits when the price of the factors of production they utilise do not reflect the true cost, then they have nothing to do in the market in the first place. The removal of incompetent entrepreneurs improves the efficiency of the economy.

Paying the wages demanded by the union would not make the enterprise unprofitable
The capitalist's decision to shut down is damaging to everyone: himself he loses his profits, the workers lose their wages, and society loses tax revenue. The reason for the action is that he looks to his class interest: yielding to reasonable demands for higher wages is a threat to future profits for all capitalists. Better then to strongarm the unions into submission.
In effect the capitalist is taking the production hostage, and shuts it down if his demands are not met. It is hardly the unions who should be blamed for the resulting loss. Bust the capitalists and not the unions.

This was a response to a write-up written by sakico, voicing the opinion that there should be no laws preventing union busting since unions are harmful anyway.