Why do some people let their car idle in parking lots for like 15+ minutes before driving off?

I keep noticing this at my gym and the grocery store near me. People will get in their car and just sit there with the engine running for forever, sometimes I see the same car still idling when I come back out 20 mins later.

Is there actually a reason for this or are they just on their phone? I always thought idling was bad for your engine plus you're basically burning gas for no reason. My dad always taught me to start the car and go within like 30 seconds unless its freezing cold out.

I try not to idle much cause I got some money set aside for a trip next year and id rather not waste it on gas or mechanic invoices, but maybe theres something im missing? Do modern cars need more warm up time or something?