2-part question involving empty commands and double-colon rules

[Robert P. J. Day]

  first, the question about empty commands.  i read that the only real
reason to define a rule with an empty command is to "prevent a target from
getting implicit commands".  fair enough.  but what's the value in a rule 
in which there are no pre-requisites and an empty command?  as in:

target: ;

  if "target" is *not* the name of a file but just the name of an action
of some kind, then one might just as well use .PHONY.

  and what would it mean if "target" *was* the name of a file?  with no 
pre-reqs and no command, what would this mean, if anything?  what 
processing could it possibly do?

  and for part two, related to my earlier posting, there is a makefile 
"main.mk" that is expected to be included by just about every other 
makefile in my project.  as it was handed to me, this main.mk file 
contains in it the following two lines:

makefile : ;
%.mk:: ;

i'm not sure what the value of these two rules are.  given that this file 
might be included in numerous other makefiles all over the project, what 
are those two rules supposed to be doing, if main makefiles are typically 
called "makefile", and secondary makefiles would be named "*.mk"?  is this 
some well-known recursive makefile idiom?

as always, thanks for the information.

rday