It is the only rule which allows you to change between a and a v. Notice that the antecedent of the statement gets negated when changing between and v. Given P Q , there are two possible states of affairs. P could be false. P could be true, in which case Q is true as well. Any time you want to work with a you will use one of these rules.
Exportation is a very specific rule.
You will use it rarely. But don't forget that it is there if you get stuck. Using all the rules of inference, derive the conclusions of the following symbolized arguments. If congress enacts a law that either establishes a religion or prohibits the free exercise of religion, then that law is unconstitutional. Therefore if congress enacts a law that establishes a religion, then that law is unconstitutional.