No, it is not just a reminder, it is a tremendous aid in interpretation.
Reverse sensing is all in your head. The aircraft or the CDI does not
know reverse sensing. Read this article:
http://131.238.38.204/~sarangan/avia...or-article.pdf





Andrew Gideon wrote in
online.com:

wrote:

If the OBS is set to the course, you can immediately discern whether
you are north or south of an eastbound course, for example. You can
do this by looking at a properly set OBS ring, and at which way the
needle is leaning.

Setting the OBS during a localizer approach is a good mnemonic. I do
that. It's also a good habit.

But it doesn't really impact *interpretation* of the CDI's display
(unless one forgets the course marked by the localizer, in which case
the OBS setting saves having to look down at the plate).

Using your example, the OBS doesn't do anything but remind you that
the course indicated by the localizer is eastbound. Useful, true, but
I'd not consider that a part of the interpretation process. I make
this distinction because, unless I forget the course, I never need to
look at the OBS to discern the meaning of the CDI.

- Andrew