Well, in Parliamentary debate and speeches in the UK it's common to interrupt the speaker with calls of "Hear! Hear!" and other formulaic phrases expressing either approval or disapproval. Different legislative bodies have different customs, and the Joint Sessions of the US Congress have been offering applause, sometimes standing, for at least all of the previous century, and possibly longer.
Clearly there is party politics at work, the party of the President wanting to show support for everything said, the opposition for little. That's why all Presidents throw in "flag, Mom & apple pie" references to force everyone to clap, especially now that TV cameras are live in the chamber.
This is as much political theatre & ceremony as it is a functional speech. It is definitely not analogous to a meeting at work. Copies went out before it was delivered before Congress, so Republicans could prepare their response which immediately followed. Key leaders already knew what Obama was going to say, and select media gets copies, too, which they pledge not to divulge before the address.
Personally, I thought it was a tour de force performance by Obama. He used all his campaign oratorical skills, in a very clear & concise speech. I don't usually listen to these things, no matter who the President is, but I found it masterful, on a difficult & complex subject.