The CET is typically set at a high enough level to protect the domestic industries of the member countries from outside competition, but low enough to allow for some trade with countries outside the union. In some cases, the CET may be adjusted over time as the member countries of the customs union negotiate trade agreements with other countries or regions.

The CET is one of the main tools used by customs unions to promote economic integration and cooperation among member countries. It is often seen as a stepping stone to the creation of a single market, where goods, services, capital, and people can move freely among the member countries.