Oklahoma! expanded on the precedent set by Show Boat, by not only being a serious piece of storytelling, but by using all its songs to do so. Every number serves a specific function in advancing the plot or developing the characters. There are no throw-aways or "cigarette songs" (as used to be the case where you could go out for smoke without missing anything important).
Now it's time to surrender to the most thrilling Broadway show ever imagined... the longest-running musical phenomenon in Broadway history. Unmask your savings today, because tonight belongs to Phantom. Tickets as low as $27!