Almost everyone making online transactions thinks that using bitcoins would be a good way to stay anonymous. But the truth is bitcoin transactions are not really anonymous at all. All transactions are recorded and can be seen publicly through the blockchain. When you use your bitcoin to pay for goods and services online, you will need to provide some private information in some cases. This means that your transactions can be traced by a third party to find information. In order to avoid this from happening, bitcoin mixers provide services where your bitcoins are mixed with others to make it harder to be traced back from the original owner.
Do you think that it is a must for everyone to use a bitcoin mixer? Why and for what purposes?
Bitcoin is pseudonymous. All transactions are permanently stored on blockchain and visible to everyone. It is possible to identify a person through their transaction history and this is where bitcoin mixers are useful, complete anonymity.
Now without any ulterior motive people use mixers to protect their anonymity. But mixers also serve illegal purposes like financial crimes including tax evasion, money laundering, fraud and then darknet crimes. Stolen bitcoins, ransomware, hackers use mixers to get rid of tainted coins.
And even using mixers isn't risk free, you have no idea who are behind centralized mixers, do they keep logs? If being anonymous is the purpose then it can be done even without mixers and if other purposes are involved then I think you need mixers.