Why do we have to care about coinjoin/mixing services?
Because
your bitcoins are being traced by
arbitrary analysis companies as we speak, in order to find any taint on them which any cooperating exchange will then freeze as soon as you make a deposit, without you even knowing about said taint.
Do you think coin mixing is the answer to this problem? If you use mixers, then the exchanges will freeze your funds again because now your coins are coming from a mixer.
Decentralized coinjoin when done with enough participants and UTXOs offers much stronger guarantees than a mixer ever could, and since all mixers addresses are being tracked by chain analysis, it is not useful when you want to send the result to an exchange.
This is almost what Samourai was doing, minus the "decentralized" part since all their coinjoin transactions were coming from a server. But if you have the ability to set up decentralized coinjoins with a reserve of something like 1BTC then it becomes near impossible to track the ins and outs of them all - especially if dozens of people with 1BTC start their own coinjoin coordinators.