Partnership is the combination of skills and experience toward a common goal. It is the shared belief in and responsibility for a product. It is a trust that if you do your best and I do mine, we can create something unique and worthwhile.
When you engage a QA partner to share the responsibility of your software project, you get to experience all those warm sentiments of cooperation and trust. But you also get to strengthen your own development engine with the skills and experience of expert testers.