Why does personal data portability between platforms matter?

Over time, a digital service may accumulate messages, photos, posts and connections. Being able to export information in an understandable format matters to people who want to close an account or move to another service. A download is more useful when its structure is documented and machine-readable.

What information would you want to take with you when leaving a service? How can platforms make portability both useful and safe from misuse? Consider the needs of everyday users as well as developers.