This is important federally, as it was important in Quebec. It was done many years ago in Quebec.
Basically, it's so there is a proper balance of power. Unions are sometimes accused of wanting to create a labour dispute. That's not the goal. The goal is to have a settlement, and therefore to have proper negotiations. The best way to do that is to have the right balance of power.
For the balance of power to be correct, it must not be impossible for one party to gain an unfair advantage over the other.
That's why we need anti-scab legislation.