Colligan was correct such an agreement would be illegal and could have just stood on that.
This is a common - and highly useful - strategy if you've just been in a meeting where things have been said that you were uncomfortable with.
Being succinct would have defeated the purpose.
This is a useful strategy to employ in all kinds of situations. A dated note written shortly after the event is going to carry more weight than recollection later. An e-mail sent to the other party that they have not contested the understanding of is going to carry more weight than just a note.
And if, as Jobs did, your other side responds, you now have ammunition for any future lawsuits.
EDIT: Creating a paper trail is a method to take note of in other contexts too. Your manager at work asked you to do something unethical? Summarise your understanding of the conversation in writing and ask if you understood it correctly (a lot of the time, the request will magically become a misunderstanding).