Security theater is most definitely not for show, and it is just as certainly not to make people feel safer.

Think about it: What does a politician or government agency benefit by people feeling more secure? Nothing. Scared people are more easily manipulated.

No, it's not for show or for perceived safety.

There's only one reason for security theater: CYA. It's so that the next time - and there will be a next time - some terrorist action takes place, the people in positions of power (who would clearly like to remain in positions of power) have ample opportunity for saying "We did everything we possibly could. We simply did not have the money/resources to protect against this. But with more funding...."

Pure CYA. Nothing more.