bitcoin transactions are recorded using the blockchain capabilities so each bitcoin can be traced to it's origin and if it was used for a fraud you would prefer to avoid the tractability of that exchange to you. here is where the mixer help you to hide the coin origin. lets say you have one wallet for all the black market activities and one for you day to day activities, when you would like to transfer money from the black market wallet to your legitimate one you would use a mixer in hopes to hide the true origin of the bitcoin. another reason might be that you want to buy something illegal and you don't want that deal connected to your wallet (or harder to connect to your wallet) You must remember that this is not a totally perfect but it's a lot safer and the more mixers you use the harder it becomes to recognize the true origin of the bitcoin you sent to the black market deal you have just done