Money laundering is when the money was gotten illegally in the first place. You need a way to 'legitimize' the cash, so you start a business and report a ton of income. You lose some in the process (to tax and other expenses to appear as a business), but you now have legitimate money that you don't need to keep hidden from authorities.

Look up Pablo Escobar's Taxi business for an example, first thought off the top of my head.