Whether you call them case studies or success stories, your customers' experiences are the best way to showcase what you can do for a prospective customer and that you deliver what you promise.
Case studies are also one of the strongest foundations for building and evolving your messaging. The active pursuit of customer details gives you a touchpoint on an evolving market's pulse. Live interviews about how they selected you, what they hoped to accomplish, their initial experiences with your organization, their longer term experiences and of course the results. The facts and the anecdotal insights as well bring the story to life.
It always starts with one story but as your library of stories builds, your sales team can select stories that suit their prospect best - by vertical, by size of company, by geographic dominance and even by what problems the prospect is looking to address.
With an ROI analysis, the client becomes articulate in communicating your value. A case study makes their story human and compelling. The interview process helps the customer to communicate what you do and what it means to them. With skillfull listening and the right questions, it may uncover areas of impact you didn't know you had, potential changes with the account and how to evolve your messaging over time.