Opinions differ a lot on this topic, whether to have one or not, if it’s more harm than use, accelerator or brake pad.
Here’s my take: In all the jobs I’ve been working so far, the number of the ones where a manual existed totals up to zero. This is because in Germany having a manual for a job is not common practice. Why? Um, maybe because the downside of a manual is that people respect paper more than people, so if your boss wants you to do something different than “your job” you could always retreat to “sorry, that’s not in my manual”, so a failure wouldn’t be your fault. Without the manual, there’s no shield for you to hide behind. Maybe. Also, making a manual requires analysis and breaking down what your company is doing step by step, and you might end up figuring that the thing is going horribly wrong and you need to do something. But it’s been working so far, and as they say, never touch a running system.
But whenever I commence a new job, there needs to be someone removed from their job to give me the rules and “this is how we do it here”. And of course, the co-workers are also very engaged to correct, update and footnote as we go along. Hint: It’s no use. Can’t keep it all in mind at once. So I’ve always sat down and taken my time to write down procedures. This can be a carthatic process, because it tells you more about where you’ve ended up than you might want to know.
The real advantage of a manual unfolds when it’s not regarded as cast in stone, but as a wiki, work-in-progress or milestone tracking tool. To say “This is how we do it at the moment, subject to change, if you feel like contributing and help us to improve our procedures, please do so. Remember, the specs mentioned in it are the lowest possible standard here, we want to do better — and we want you to do better, since we hired you because we believe you will do better”. Wouldn’t that be great?