Answer -
There are many a times when we need to use written communication only .
Some of them are-
1.Where the information has to be stored and retrieved .
Example say official records.
2.Where the reliability and validity and authenticity of an idea is of utmost importance .
Example say decisions that are taken by a group .
3.Where the verification and authentication of an idea is of utmost importance
Example could be of say a memo from boss to employee for new rules to be followed.
4.Where objective references are important
Example is of research article
5.Where written communication supplements oral communication.
Example The minutes of a meeting.
6.Where decisions are of crucial nature and actions are to be considered.
Example the plans and proposals that are made in course of the business.