Why are we supposed to tell service members, “thank you for your service”?

I don’t understand it. My dad served in the Air Force. He got paid a salary, a place to live, free healthcare for life, preference when applying for jobs, and they can buy USSAA insurance or something.

I hear that a lot and I was just curious was prevailing thought was on this subject.

Remember I’m not saying people shouldn't say that. I’m asking why.