Two things rule the reliability of a system. Application and infrastructure. Developing applications is not easy but complicated and can be controlled to a certain extent. Things we know works are called features and things we cannot or forgot to fixed will be called known issues. There are many tests you can run to ensure the code works and code + UI = application works. Unit tests, UI tests, automated scripts, you name it.
It is one thing to have a working application. But how about infrastructure? Many thought cloud could make life easier for infrastructure people. It just made life even harder. If things go wrong (chaos happens), there can be very minimal we can do to control. One thing we can do is try to plan for chaos.
Azure Chaos Studio just allows you to plan and test your infrastructure and prepare to be ready for any unexpected situations. It is not just a signal based simulation. It makes infrastructure fail for real. This makes the worst possible scenarios testable.