Let's say you only want to calculate the most recent month loaded to the database.
Something like:
      Fix (January)
              calc dim (accounts, actual, geography, location);
      endfix

Instead of opening up the calc script each month and changing the month by hand, you would have the following:
      Fix (&CurMonth)
              calc dim (accounts, actual, geography, location);
      endfix

Then before you run the calc you would update the substitution variable on the Essbase server.

This doesn't seem to provide any savings at all.

Well, what if you had 30 calc scripts where each on referenced specific months that are changing over time.  If you set each script to use a substitution variable, instead of having to open and change each of 30 scripts, you could merely change the substitution variable in one place on the server and all the scripts would then be updated.

A substitution variable can contain more than one value.  You can explicitly reference members separated by a "," use a range where you separate members by a ";"
&FcstMths  = (Aug,Sep,Oct,Nov,Dec)
or
&FcstMths (Aug:Dec)

Then the usage of this substitution variable could be:

Fix (&fcstmths)
    clearblock all;
endfix