A personal development plan sets out where someone is now, where they want to get to, and how they'll get there. It's a way to measure progress rather than a form to complete — acknowledging strengths, naming weaknesses, setting realistic goals, and identifying the resources that will support growth, whether that's mentoring, courses or hands-on experience.
The value is accountability. Without a plan, people move from one goal to another without finishing any of them. With one, effort has a direction and progress can actually be reviewed.