PEP 572: Add additional rationale from while loops

pep-0572.rst

@@ -27,6 +27,11 @@ are currently several ways to spell this, none of which is universally
 accepted as ideal. A statement-local name allows any subexpression to be
 temporarily captured and then used multiple times.
 
+Additionally, this syntax can in places be used to remove the need to write an
+infinite loop with a ``break`` in it.  Capturing part of a ``while`` loop's
+condition can improve the clarity of the loop header while still making the
+actual value available within the loop body.
+
 
 Syntax and semantics
 ====================