Why? Rather than write these down and share asynchronously with potential hires, use your real problems to inform interview questions. At worst you've spent time brainstorming a problem that you were already going to be thinking about, and at best you're getting fresh perspectives to help solve them. However, while you're hiring someone to solve today's problems, there is a never-ending stream of new challenges that follow. Companies often skip over the potential for growth that new hires will have, because it's challenging to predict how the role will change. Being transparent about how the role will shape and evolve gives top performers comfort that when they're skilled and collaborative, they will have the potential to shape a role that suits them as they grow individually.
Where? Fold them into your interview script.
Startup Example: Quorum includes a section in their posting that speaks to your job after this role. It details how the role might evolve, the different avenues that this hire could choose to pursue, and recognizes that entrepreneurial early hires might choose to launch something of their own.