What could be a reason for using a mixer if everything is legit? Just curious
The most obvious reason for me is because i don't want anybody to know how much BTC i own exactly. Most people keep their private keys on a desktop, a hardware wallet or a paper wallet, but they still keep everything in their home.
If somebody would know you helt the private keys of addresses loaded with 100's of BTC in your home, they might get tempted to rob you...
Also don't forget that at the time you withdraw funds from an exchange, it is linked to all of your personal information. People quite often don't realize that, but it's the hard reality.
The exchange in question from there can follow everything you do, where you're spending your coins, what other exchanges you are connected to, and the list goes on.
What's even more worrying, is that governments can do that too if an exchange gets forced to hand over every detail of their customers. Mixers help people with maintaining at least some form of privacy.