When asked questions about testing or design patterns on interviews, I respond with but what we want to achieve? Tests are not masturbatory method of improving

When asked questions about testing or design patterns on interviews, I respond with but what we want to achieve? Tests are not masturbatory method of improving your self-esteem as developer, but a method to write robust code, easy to refactor or modify without breaking unexpected things. Any approach to testing should be motivated by business use case - does this approach actually lower expected future development cost in terms of fixing bugs and adding more functionality? How much it will cost if this part of system breaks?