The State of the Union is required by the Constitution but over the years its form and function has changed drastically.
The reference in the Constitution is brief. Article II, Section 3 of the United States Constitution reads, "He shall from time to time give to Congress information of the State of the Union and recommend to their Consideration such measures as he shall judge necessary and expedient." However, it says nothing about a speech or the timing of it.
The State of Union has become an annual address given in January or February, by practice. Presidents in years past (before Woodrow Wilson, pictured) have submitted written reports to Congress to satisfy the Constitutional requirement.
Presidents now use it as an opportunity to hammer home their priorities. With the rise of mass media, the State of the Union has gone from a matter of routine to an event akin to inauguration.