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.
Haven't you ever thought that exchanges require KYC and every transaction/log/activity that happens from your account is recorded and what's even worse is that it's done by a regulated service provider, something that has common with government.
While mixers require zero KYC, at least this is guaranteed, we can't 100% surely say that they don't keep any log but for example if we remind bitmixer, it was the greatest and biggest mixer ever, top with anonimity and very trustworthy. Also to add, you can use tor link for better anonimity, none mixer will ask you about fund sources and etc, all you do is deposit and get, without problems and traces.