Most money that people make illegally (selling drugs, etc.) is cash, because you can’t trace cash transactions. The problem is that it’s hard to use cash, and just depositing vast sums of money in the bank raises the obvious question of where the money came from. At some point, even if they can’t prove that the money came from anything illicit, they can prove that you aren’t paying income taxes on it, which is a federal felony by itself.

So you need to operate a business that also deals largely in cash, slip your illicit cash into the businesses’ revenue, cook the books and make it look like the money came from your legitimate business and not your illegitimate one, and Bob’s your uncle, at least in theory. In practice, legitimate cash businesses are more and more rare as time goes on, and investigators are really good at noticing when some random nail salon or bar has a lot more revenue than they should.

Also, don’t be stupid enough to operate your illegal business out of your legal, money-laundering front. Seattle used to have an extremely sketchy but extremely good cash-only teriyaki joint downtown, but they got busted for fencing stolen iPads after hours. Which explains a lot about why they never accepted credit cards, I guess, but they were a thriving teriyaki joint and I kind of miss them.