The president delivering the State of the Union address in person is a relatively recent phenomenon. Before Woodrow Wilson restored the practice, even populists like Andrew Jackson and Theodore Roosevelt fulfilled this Constitutional requirement by sending an address to be read to the Congress, which is curious, since the State of the Union is the president's most important speech, both substantively and symbolically. It gives him the opportunity to set a governing agenda, a chance to grab the commanding heights at the beginning of a legislative year. With all of the Congress, president's cabinet, justices of the Supreme Court, and Joint Chiefs of Staff arrayed, it theatrically reinforces that our executive is the primus inter pares of our political system.
This year's State of the Union message will be especially important for President Obama, since a new Congress has just taken office after an election widely considered a referendum on the first half of the president's term in office, and the opposition has an activist agenda that, if adroitly implemented, would effectively sideline the president for the coming two years.