Why would I use "Both" COM threading model instead of "Free"?

According to this article if I register my COM object with either "Both" or "Free" threading model that object must be completely thread-safe. Specifically all accesses to global shared variables must be synchronized and all accesses to member variables must also be synchronized. That's a lot of effort.

Now I understand that being able to register my object as using "Free" threading model is advantageous and might be worth paying the price of making it completely thread-safe. But why would I want to do all the same and register my object using "Both" threading model instead? What would be the advantage? How do I choose between "Both" and "Free"?