Apparently I'm able to net use * http://servername.com and this concept is completely new to me.
Does this mean I'm using WebDAV?
What are the security implications?
How can I read/write files in this manner?
Why doesn't it work for every website?
Is it possible to write files using this method?
Yes and no. After this net use statement, you can read regular http content as though it were files on your local disk.
For writing/random access and "dir" support, the website needs to implement additional webdav commands.
Yes, you are using WebDAV.
On Windows XP and Server 2003 "net use" supports only NTLM and Kerberos authentication. On Windows 7 and Vista you can use Basic via HTTPS, Digest, NTLM and Kerberos are supported. I would suggest taking a look at this article: http://www.webdavsystem.com/server/access/map_drive
After "net use" you will have a drive mounted in your local file system. Now you can create, delete, copy, move, read files and folders. Just use any file manager, command line tools or any API that works with file system.
Most websites does not provide WebDAV access.
Yes, you can write files. Just use any API or software that can work with local file system.