While you are in the midst of the engagement, you have all the details clear in your mind. You don't need notes to remember the name of your go-to person in the London office or name of the process that has to run before you run the daily report.
Write it all down anyway.
Next month or next year, when you've worked on three other projects and replaced that information in your memory several times, you'll get a call from the client asking you to come back and "make a few simple changes".