Answer:
1. The first reason is that joining in an international or regional organization can benefit them the opportunity in economic development with the policies of the organization. For example, when an European country joins the EU, they would benefit the free movement of trade, capital and labor. By which, they can foster the activities of production, export. In addition, they can also benefit the trade agreements with different countries, regions other then EU thanks for the agreements between the Union and its partners. For example, all EU member countries can benefit from the trade agreements between EU and Canada when having a marketplace to export to.
2. The second reason is that they can be avoid the consequences of conflicts under the protection of the international organization. A typical is the member states of United Nation as this organization has the goal to maintain the peace and security among participating countries.