I think speed is one of the primary advantages in using email over a traditional business letter.
In a business setting, it is desirable to receive immediate clarification. The longer an issue goes unresolved, the more difficult it is for business to proceed. Email provides an immediacy that a traditional letter simply cannot. For example, if you were a manager at a factory and you had a question on production specifications, an email to your design branch would be essential. You could get your response and quickly proceed with production. If you wrote a traditional business letter, you would be waiting for a response, and the wait time would slow up production. This would inevitably impact financial profit. In the example, email is a quicker medium to compose and send a message, and an email is likely to receive a faster response. When it comes to immediate questions that require clarification, email is preferable to a traditional business letter.
In business, primacy is placed on speed because of the need to facilitate decisions and the importance of taking decisive action. Email provides speed. In this way, there can be a significant advantage in using email over a traditional letter.