If you build a mini business empire and it subsequently fails, a small consolation prize might be a net operating loss carryover that will shelter some or all of your more modest income for many years. Of course NOLs are only one among many carryovers that need tracking. In my experience the tracking often leaves much to be desired. Changes in tax preparers or even software can result in the loss of valuable carryovers. But that is not the worst of it...

How Should a Disregarded Single Member LLC File a W-9?
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.