They don't market themselves as a mixer, but effectively that's what they offer. Their purpose is to obfuscate funds.
What "they" offer, is a privacy-orientated wallet. We have way too less of such wallets implementing new features and improvements.
CoinJoin is a decentralized way to regain some level of anonymity by mixing funds with other participants.
You are free to use your SPV wallet which communicates with a 3rd party server telling them all of your addresses, the amount of BTC you own, all your transactions, your IP address and therefore a relatively precise geo location.
But not everyone has to use such a privacy-invading wallet. And IMO wasabi actually is the best privacy preserving SPV desktop wallet available.