LJ Braintrust Question

I believe my friends list contains a couple of people with an education in law.  For those of you who might be knowledgeable in such things, I ask:

Many legal documents currently sent via courier (often at exorbitant expense, particularly for documents that need to be somewhere in under a day) rather than via a seemingly-more-convenient (faster, cheaper) electronic means such as email or fax.

• What sorts of documents are sent this way?
• Why is an electronic method not used?  Specifically, is it because of a limitation of the medium (i.e. worrying that an email could come from a false source, or be tampered with, or be corrupted or lost, or that the sender might later claim it was faked or not sent by him), or a limitation of the law (i.e. the particular document in question has some law that specifically says it must be printed on dead trees and couriered?)