One of the things that bothers me the most where I work are people that won't document what they do in fear that their employer will keep their documentation and replace them with a lower cost employee. These people don't realize that having or not having documentation will in no way stop their employer from firing them with no reason given.
Back in the day when I worked at Dell, accountants decided that Dell needed to reduce their workforce by 10%. At the time, that meant that 8000 people needed to be laid off. So to make it "not personal" they decided to select a specific pay grade, and then lay off everyone in that pay grade. It had nothing to do with work performance. I saw people (myself included) that produced millions in revenue for Dell, lose their jobs.
So if documentation (or the lack thereof) doesn't contribute to job security, what does it do?
Documentation ensures that processes are efficient, consistent, and scalable. Documentation ensures critical business processes survive the loss of personnel or resources.
Documentation ensures that you can go on vacation, and that the person that has to do your job while you are away, does not develop a personal dislike for you. It also keeps your phone from ringing on your time off. Most importantly, unless you have a photographic memory, documentation helps you keep knowledge organized and available when you need it.
Make the time to document your business processes. You will be glad you did.