1) Accuracy of submission. No keypunch operator to transpose or put numbers on the wrong line.
2) Electronic and trackable Acknowledgement, usually within 1 or 2 days.
3) Quicker processing of the returns.
4) Many states only allow Direct Deposit with e-file.
5) Less postage, paper, toner etc.
6) Less time copying, colating, stapling, trying to get 13 pages into a #10 envelope.
7) No mistakes about where the return gets sent.
8) No doubt if the return was "sent" or not.