Under an RTA, tariffs fall on imports from the other members of the agreement, but they need not change on imports from non-members. As a result, RTAs imply both trade liberalisation and trade discrimination. Whereas there is a near-consensus among economists that the former is desirable, that is not true for the latter.
Trade liberalisation within a trading bloc tends to be beneficial when it promotes a shift of resources from inefficient domestic suppliers to more efficient producers within the region. Economists call this phenomenon ‘trade creation’.