Your accountant prepares your tax return for free. They also have a lot of boxes of flyers provided to them by people who want to sell things. After preparing your tax return, they use their knowledge of your return to choose which flyer to put into the envelope. They then send that envelope back to you, and when you open it to read your tax return, there's a flyer for something else paperclipped to the front with a note saying "Thought you might find this interesting."

(In particular, the accountant is only one who sees the information in your tax return.)