I was on a UA flight many years ago, soon after the UA flight that crashed in Iowa. Many people clapped, which I thought was rather stupid. Several other flights in various parts of the world where it seems to be a custom for people to clap.

Other than for such reasons as mentioned - inaugural flights, wishing a pilot a farewell, or if the flight was in obvious extreme danger and the crew did an outstanding job of safely landing, I don't see the need and think of people who do as being somewhat simple minded. I never join them and simple chalk it up to inexperienced flyers who think that something extraordinary has happened when it actually is nothing but the ordinary.