It lets people communicate in non-real time. At their convenience - not when they are interrupted. Reduced labor cost - Instead of answering phones and taking messages employees are free to do more vital tasks. You receive all your messages and can reply immediately to the correct party. Improved message content.
Voice mail is much more accurate and private than pink slips.
Messages are in your own voice, with all the original information and inflections. Less paging and holding time. Less peak load traffic. Twenty-four hour available. Better customer service.