Bitcoin itself makes money laundering easier. Say, you have a few million dollars of cash from drug trafficking and you need to conceal the origins of the money obtained in this way as well as transform the proceeds of such an activity into legitimate assets. You start buying bitcoins for cash, for example, using LocalBitcoins userbase. 1M dollars equals roughly around 1,500 bitcoins at the moment, therefore it may take some time and surely is not the most optimal way if you want to make this conversion. Nevertheless, after you convert your dirty dollars into bitcoins, you just sell the latter at some exchange and get clean dollars transfered to your legitimate bank account. You may even want to pay an income tax if you consider yourself a law-abiding citizen of sorts.

Disclaimer: I am neither engaged in nor encourage any such activities