A main purpose of the president's State of the Union address is to

A main purpose of the president’s State of the Union address is to identify major policy issues.

Explanation:

The formal basis for the State of the Union Address is from the U.S. Constitution:  The constitutionally mandated presidential message has gone through a few name changes:

t began to be informally called the "state of the Union" message/address from 1942 to 1946.

Since 1947 it has officially been known as the State of the Union Address.