Under normal operation of CM, the mapping from path names to the files they denote is supposed to be a fixed one. The path anchor mechanism is merely a means of configuring this mapping according to the actual filesystem layout.
The bootstrap compiler and the associated boot procedure, however, use path anchors extensively for other purposes. In particular, the above-mentioned mapping will vary over time.
The ``casual'' compiler hacker does not actually need to remember all the details because these details are mostly taken care of automatically by CMB and its various scripts ( makeml, testml, installml). But it is useful to have a rough idea of what rôles are being played by various files and directories that are being used or created.