So, Mixers provide service of shuffling bitcoins or provide addresses loaded with bitcoins along with private keys. In short, they provide service so to create anonymity between sender's and receiver's address so that no one can trace the transaction.
My question is why not use Exchanges?
If you are planning to use exchanges and other platforms to wash the coins then you might get your coins and account getting a ban, you cannot deposit money without trading and then withdraw doing nothing and if you are doing that with huge amounts your account will be restricted and you need to verify your identity, so if you are looking for privacy then mixers are the place to go which are dedicated services for privacy.