I thought the same thing as you and had the same reservations when a marketing exec asked me to put one on the purchase page for a startup I used to work for. I was shocked when the A/B test we ran showed a lift. I wish I had more details but this was about 5 years ago.

I think people just want some assurance, even if it isn't backed by anything they understand, that a shows them that a decision they really want to make is ok. Putting a shiny badge that says it is safe is enough to sway some people.