The other URI schemes (except nntp) share the property that they are equally valid at any geographical place.
There is however a real practical requirement to be able to generate a URL for an object in a machine's local file system.
The syntax is similar to the ftp syntax, but in this case the slash is used to donate boundaries between directory levels of a hierarchical file system is used. The "client" software converts the file URL into a file name in the local file name conventions. This allows local files to be treated just as network objects without any necessity to use a network server for access. This may be used for example for defining a user's "home" document in WWW.
There is clearly a danger of confusion that a link made to a local file should be followed by someone on a different system, with unexpected and possibly harmful results. Therefore, the convention is that even a "file" URL is provided with a host part. This allows a client on another system to know that it cannot access the file system, or perhaps to use some other local mecahnism to access the file.
The special value "localhost" is used in the host field to indicate that the filename should really be used on whatever host one is. This for example allows links to be made to files which are distribted on many machines, or to "your unix local password file" subject of course to consistency across the users of the data.
A void host field is equivalent to "localhost".