Do I need to write this?
It's one of the questions that you need to ask when writing e-mails. In fact, it is a question that you need to ask before doing anything - is there a need for this?
It is important to ask the question because e-mails should be concise and to the point. It shouldn't be superfluous (read: wordy, too many things said, unnecessary information, more than enough). So whatever is said in the e-mail should inform the reader what is important - if you're still scratching your head: why you're writing the e-mail.