Agile approaches aims to improve the collaboration between the development team and the end-users or the Product Owner in Scrum. As far as software testing is concerned, it is however deceptive to believe that this could happen without a strong contribution from software testing experts.
In a test-driven approach, when developers pick up a user story to work on, the output of this conversation with the customer should include a set of tests that precisely specify what’s required.
Customers are usually not software testers, so we must offer them guidance on this process and help them to identify the test scenarios we’ll need to consider (e.g., if they ask for new library members to choose a password when they join, we might ask the customer to consider what should happen if the password they choose is too weak, or what should happen if the password field is left blank, and so on.)
Teams that expect customers to go away and write the tests themselves could be waiting a long time. This is a technical skill that takes a long time to master. If you have dedicated testers on your team, this is one area where they can prove very useful, helping the customer to articulate their needs as tests.
Source : TDD, Jason Gorman, http://www.codemanship.co.uk/tdd.html