> I sometimes come across stationary cars with a running engine, and I honestly wonder why anyone does that.

As someone who frequents very warm countries, I think most people do it because of the AC. If the engine is not on, the AC won't adjust the temperature that you've selected.

As a general rule, most machines break during state changes like start/stop. Keeping an engine running means no piece is subjected to the extra acceleration of restarting and your starter isn't used.

A good portion of the mechanical wear and tear on an engine comes from starting/stopping it.

If you grew up poor a few decades ago, you probably saw your parents begin every journey praying the car would start. Because loads of vehicle faults manifest as failing to start - flat battery, spark plug problems, cold weather, no fuel.

Starting the engine is a stressful experience if you're used to vehicles where sometimes the engine won't start. Especially if you're at the front of a queue of traffic.

It's actually an offence in the UK to do that. Not that it stops anyone.