Strikes stop production--when unions use a strike it stops production of the product or service being created. Though many strikes were broken with new workers, this is not always possible at least not right away. Therefore, the business economically suffers making strikes effective.
Strikes make your goal publicly known--strikes put the issue of the union in the public eye. By using a strike, there is a visual to a company of workers walking out and picketing. Other members of the community become aware of the issue and can participate or support the union. The strike can also inspire others to join or begin their own strikes.