The requirement to use a particular format for your copyright notice was a vestige of the now-obsolete Universal Copyright Convention. As per Wikipedia,
U.S. law no longer requires the use of a copyright notice, although placing it on a work does confer certain benefits to the copyright holder. Prior law did, however, require a notice, and the use of a notice is still relevant to the copyright status of older works.
The precise format required by the former U.S. law (before March 1, 1989) is detailed in the second link, but basically it's © Year Name or equivalently Copyright Year Name if the designated copyright symbol is not convenient to use.
The year is important so you know whether the copyright term has expired. The name is important so you can figure out who the owner of the copyright is. While email would probably serve the same purpose, the law predates the existence of email by a number of decades.
The other formats you see are probably either (a) a notice by someone who just couldn't be bothered to look it up, or (b) something required in some other jurisdiction at some point. Probably there would perhaps occasionally be somebody unaware of the former law who thought they could "improve" on the standard format specified in the legal statutes.
I am not from the United States and if I was, I would not be certified to provide legal advice there.