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? When we deposit funds on exchange, bitcoins get deposited on exchange's cold wallet. But when we withdraw funds, withdraw gets processed from exchange's hot wallet. So in short, it is very easy to do anonymous transfer by depositing funds on exchange and withdrawing to other addresses. Why to pay 1-3% fees on Mixers when it could be done for much less on Exchanges. Exchanges like Binance charge just 0.0005 BTC to process withdrawals.
Choosing between exchanges and mixers depends on your needs such as the level of anonymity you prefer. If you were just an ordinary crypto enthusiast who is not privacy centric and doesn't a need to hide any crypto transaction you might have, then you can choose the service of an exchange, otherwise, you need to use the services of Bitcoin mixers.