Although Form W-9 has some other purposes, a company will usually ask you to fill it out if you are a business-to-business service provider . If you don’t provide a TIN to your customer, they may have to subject you to backup withholding . If it is my company that asks you for the W-9, I am not going to do backup withholding if you don’t send me the W-9. I’m just not going to pay you until you cough it up.
I believe in learning from experience. But it is better if the experience you learn from is other people’s experience. That is why a very large percentage of my tax writing is from court opinions, most often the United States Tax Court. This piece is my own experience.