Make a little experiment, leave your paper notebook and a mobile or laptop on the table in public and leave for 15 minutes. With 99% probability I think your gadget will be stolen, 1% that both items will be stolen. When theft get your gadget, no matter if you have used offline or online based app, theft would try to bruteforce it. Even if you did not store password anywhere on your device, theft could search for other valuable things. Passport scans for example, access to emails, anything that might help him to get anything. With paper password what can you lose? No one would even care about it.