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.
You can't use an exchange as a mixer because of the limits set by the exchange. If you will want to increase your limit or remove your limit you will have to verify yourself. This means that you will have to provide identification documents to the exchange. So you can eventually be tracked anyways. As far as I know, in a mixer you don't have to provide kyc stuffs and you also don't have any limits.
it comes down to whom you are trying to hide from when mixing your coins!
for many users it is to prevent any random prying eyes from snooping around in their pocket to know how much bitcoin they own and where they spend it at. and since most of them already have accounts on some exchange site for buying bitcoin or trading altcoins then using that same account for mixing some coins makes a lot of sense.
although it may not sound like it but using mixers is a lot safer than using exchanges! there has never been any cases of a mixer scamming any user ever (there are scam sites calling themselves mixer but that is different) but there has been lots of cases of popular big exchanges scamming their users and robbing them!