I should be amazed at how prevalent this is but after almost two decades in IT/IS, it's no more than the equivalent to the Post-IT on a monitor, but more accessible. Dumb, but business as usual.
Writing passwords down on a piece of paper, and keeping that in your wallet or locked desk drawer is actually one of the more secure ways of storing passwords these days.
No risk of electronic compromise, and its highly unlikely that people who would steal your wallet or break into your home are also interested in your online accounts.
1. New guy gets hired.
2.You work for a large corporation and we'll you can't say you can trust everyone there.
3. Small company of 10 (company I work at for example) could be compromised by the weekly janitor.
4. Someone could break in and make it look like a robbery all while stealing your critical infrastructure.
I recently helped research a bit about internal security for our office and sticky notes are still a very common place for credentials to be compromised.