the written material is much more formal than just talking. it takes a bit more time, but works more effectively.
very good experience, and thanks a lot for sharing it.
Spoken English and written English are both equally important.
Usually written document with delivery confirmation (i.e. fax or registered mail, and even email nowadays) can be used in court as proof of notification. That will make every manager in every company take it very seriously. So it is a good choice for any formal communication.
Spoken English is what you use daily with your superior or colleagues. Usually 70% of human communication is done face to face. Not being able to use this tool efficiently and effectively will not only disqualify you for any managerial promotion, but also hinder your work efficiency.