This is also interesting. From the VFP Help on TMPFILES:

Specifies where temporary EDITWORK, SORTWORK, and PROGWORK work files are stored if they have not been specified with any of the other options. Because work files can become very large, specify a location with plenty of free space. For faster performance, especially in a multiuser environment, specify a fast disk (such as a local disk). Default is the startup directory.

So if you can specify TMPFILES, you need not specify the other 3 if the location of TMPFILES is acceptable.