Its widely described in cryptography literature not as a security device but as a literary one.
Atbash mostly survives in Jewish Kabbalahs, mystic and allegorical writings. Two atbash phrases are used in just three places in one book of the bible (Jeremiah), and these were probably added early on but not in there originally (as they don't occur in the Septuagint, for example).
Here's The Code Book (Simon Singh) description:
"Atbash and other similar biblical ciphers where probably intended only to add mystery, rather than conceal meaning"
An example of this additional meaning is the word "Mitzvah" is (mem-tzadi-vav-heh) is usually translated commandment. The name of God is Yud-heh-vav-heh. The atbash for mem is yud and atbash for tzadi is heh. So if you replace the first two letters of "mitzvah" with the atbash conversion, you have the name of God. This is an allusion that doing a mitzvah reveals God in the world, though in a somewhat hidden way.
Knowing how Bible poorly got the other science bits wrong, that's a significant thing. The writers used at least a real substitution cipher (even if it's a kind-of-simplest we know of today).