It's probably a simple release of tension by the passengers, for the passengers.

This is a thing?

I always appreciate a good flare.

I guess at the appropriate time after sliding down the slide and getting clear of the plane I'd golf clap for the pilots and thank them for saving my life.

After a landing where nothing fell off, broke, or caught fire? Nah. I thank the captain and crew on the way out and that's good enough.

It's weird. But less weird then when people clap at the end of the movie, when the actors aren't even present.

Then there's Chuck Yeager's opinion about landings...

"If you walk away from it ... ?"