Why does everyone say "get it in writing" when accepting a new job. Is this common practice? What does it mean?

What does this mean exactly? Is there a formal document that is usually sent out when a new position is offered? If this isn't automatically sent out, what would you even ask for? Call the HR department and say "I'm not accepting this job until I have it in writing?".

I know my company doesn't do this and, as someone looking to move to a different company for the first time in a decade, I want to make sure I do things right.

I apologize for my many ignorances regarding this topic.

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