Then read the Bitcoin white paper again, section 10 Privacy. It states:
The necessity to announce all transactions publicly precludes this method, but privacy can still be maintained by breaking the flow of information in another place: by keeping public keys anonymous.
A Coin Mixer can benefit this goal, since through exchanges it is possible that you can actually decrypt the owner of a public key. Coin Mixing prevents that.
What does even "decrypt the owner of a public key" mean? Your public key exposed when you spend your Bitcoin. Besides, Bitcoin was never anonymous to begin with. Bitcoin is only pseudonymous.