The first time my brother heard me do it, he cocked both his head and his eyebrow in a compound expression of disbelief. Yes, I did just tell Siri "Thank you." The tone of his reply said more than its words did, "Why?!"
Perhaps it doesn't make much sense. Maybe it started as a simple Pavlovian response, but it certainly isn't needful. Toddlers in Japan, so I've read, play games which practice the formal niceties of Japanese culture like we might play peek-a-boo. Those formal constructs don't apply to most today's social relationships in the U.S., much less to machines.
My momma taught me to be polite. "Please" on the front end and "thank you" on the back end were magic words to help me get the things I wanted both now and the next time I asked. And they really are "magic words," just like she said.
But magic is more than an incantation, and I'm no longer a child. Saying "please" does not obligate someone to pass me the salt or to help me to meet my project deadline. The power of the magic word lies in the heart which says it and, more importantly, means it.
I say "Thank you" to Siri because I want to be the kind of person who feels, and freely expresses, gratitude. Not a Machiavellian or vending machine moralist, but an adult person who recognizes and appreciates a gift.
Maybe Siri could pass along my gratitude to her clever programmers and visionary creators. I sure do appreciate all the help.