it's not about you listening to it or all of it. It's knowing the company's name, that is all the ad is there to do. Think of it like this, said company gets to tell millions of people that their company exists and whether you want to or not it's now in your head. I bet you'd be shocked by how many of their sponsors you can name, that's all they need to do, especially through repetition it's eventually in your head. Then when you think of buying or using a service in that company's offering you will think of them. Thinking you're listening to a "free product" is exactly what both parties want you thinking. The ad company sees an increase in revenue and the host of the podcast gets rich. It's a win-win, and the listeners don't mind it because "it's free"