In order to trade with another country or countries goods and services have to meet agreed standards, regarding quality, safety etc. Each standard is set out in detail, agreed by relevant experts who represent the trading partners. Rather than this work being duplicated over and over for each pair of trading nations, the EU acts collectively in the interests of all. This saves a huge amount of time, money and beaurocracy.
As the EU is the world's largest trading block, our nearest neighbour and represents more than 40% of our trade, we must continue to meet these standards in order to trade. At present we take part in setting the standards. If we leave the EU we will still have to obey the rules or lose business. We will have GIVEN UP CONTROL.