1. It puts them in control. If they want to leave the team, they have a handy list to hand to their new manager right away.
2. They can capture details I would miss, no matter how close to the work I am. They will capture exact why that design process was hard and what it was like dealing with those 4 external teams.
3. It improves their writing and communication skills. I spend my 1:1's going through the accomplishments and working with them on how to expand and add context to items, where to add detail, and how to be concise with the results.
So far, I receive positive feedback on this. On the flipside, I don't do this for myself :)