Let's take the credit card companies as an example for this explanation. When a credit card is reported stolen, the service provider <Bank> are notified and the credit card is blocked. They are able to do that, because they have 100% control over that technology.

Now, let's apply this scenario to a Bitcoin address, where major merchants can be notified to block any transaction, that are coming from that address. <Think of Supermarket chains and Fast food franchises and also fuel companies.> Let's say you donated money to Wikipedia and your government does not like that, well they can link that address to you, if you re-use that Bitcoin address at any of these companies and they can push that information to a wide selection of other merchants and services that accepts Bitcoin payments and order them to block payment received from that address. <or any address linked to that Bitcoin address>

Ok, that might sound extreme in the 1st world countries, but a lot of dictators in 3rd world countries will not hesitate one second to do that to their citizens, if they had the technology enabled to do that.

Also, if people knew your Bitcoin address and they linked that to your personal information, then they will be able to gather a lot information from that one address. They will know how much money you have invested in Bitcoin and also what your spending habits are and even where you live. <You buy from a ecommerce site and they courier the goods to your physical address.>

This information in the wrong hands, can be very dangerous, so YOU have the right to hide that information by any means possible.  This is where the Mixer services comes in handy. :->

Is this legal? Probably not in most countries, but it just gives you the same level of anonymity, similar to when you withdraw cash from a ATM and spending it randomly at merchants.

Does this help criminals to hide their identity too? Yes, but as we have seen with the Internet, technology comes with it's advantages and disadvantages. The internet are used to distribute child pornography and enables terrorists to communicate and it is also used by money launderers to launder their dirty money, BUT it is used a lot more for legal things like emails browsing useful information ecommerce social media etc.

Mixer services should not be branded as a tool for criminals, but rather be applauded for it's contribution to our financial privacy and protection from all the criminals and Con artists out there.   

These are just some of the reasons why we should use Mixer services to protect our financial privacy, there are lots more. {Financial StalkingBribery/Selective Marketing based on individual spending habits/Kidnapping  to name a few}