They make it much harder for anyone to trace your money on public blockchain protecting your financial privacy and making your Bitcoin more fungiblet. While no system is perfect against advanced tracking these services increase  cost and difficulty for those trying to spy on transactions making it low chances of succeed. It is not just about mixing. using wallets like Samourai with Stonewall and carefully managing Bitcoin (UTXOs) are also good. Many experienced users combine these methods and use other privacy tools like VPNs to keep transactions as private as possible.