Sometimes there are ideas and concepts behind his songs that deepen another aspect of the show. The song itself may seem like a throwaway, which is why the title of this thread creates a skewed discussion. ALL of the songs mentioned in this thread work in the context of the show. We can dislike them as songs, but we probably would like them better in the context of the show.
If Sondheim himself were answering the question, he might say (as he said many times) "I Feel Pretty," because the "Noel Coward-ish" lyrics, he says, are false character writing for a young girl newly arrived in New York from San Juan.
But in the context of the show, the song invariably gets a big hand. It opens the second act, and the first act ended with two deaths, which was highly unusual for a musical in 1957. The audience appreciates the frivolity, knowing they are in for more death.
If Jerome Robbins were asked the question, he would say "One Hand, One Heart"--or "The Cigarette Song," as he used to call it.
Whenever he was watching the show and that scene would start, Robbins would quietly file out of his seat, up the aisle and into the lobby, where he would enjoy a cigarette (back when you could still smoke in the lobby) before returning to his seat for the Quintet and the Rumble.