First of all, please understand that the European Union is a group of countries. A business cannot join the EU, only a country can. So since you are asking this about a business, I assume you are asking why a business would want to start making its product in the EU. I have changed your question to reflect this.
A business would want to make its product in the EU because the EU is a large and important free trade zone. This means that a product made in one country of the EU can be sold in any other EU country without having to pay tariffs or to undergo any customs inspections. The EU countries, when combined, have the largest GDP in the entire world. That means that a firm that is producing its product in one EU country can sell that product anywhere in the largest single market in the world. This is the main reason to produce in the EU--the fact that it gives access to the world's largest economy.