When the acceptance tests are elaborated jointly the testers, analysts and developers don’t have different understandings. When we elaborate them before we build developers can build the smallest and simplest thing that will solve the problem. When we elaborate acceptance criteria at the same time we elaborate requirements we don’t have as much change control required and we actually get better requirements to communicate.

via Getting Testing Involved | LeadingAgile.

Great overview of how testing works in agile and why getting testing involved at the beginning is so valuable.