I'm quite excited about this feature, being paperless here since 2012 I should have thought of it already. If you're maintaining paper copies of agreements, membership forms or signed terms and conditions, you might find this useful!
Instead of - or as well as - keeping the paper, you can upload documents into a customer's account, making it really easy to find them if you need to. No more manual searching through paper copies!
To start using this feature, turn it on under Admin > Document storage. When you do, a new Documents tab is added to each user.
For full details, see the manual page for Document storage.
Many thanks to Shellie at Flight Sport Aviation for suggesting it.