Add more rationale, as requested by Paul Moore.

pep-0383.txt

@@ -54,6 +54,11 @@ possible. The uniformity is achieved by using specific encoding
 algorithms, meaning that the data can be converted back to bytes on
 POSIX systems only if the same encoding is used.
 
+Being able to treat such strings uniformly will allow application
+writers to abstract from details specific to the operating system, and
+reduces the risk of one API failing when the other API would have
+worked.
+
 Specification
 =============