Necessity of synchronized on local variable

Necessity of synchronized on local variable

In the JSON-java library (org.json.JSONArray) I have found this code
snippet with a synchronized block around a method-local variable
public String toString(int indentFactor) throws JSONException {
StringWriter sw = new StringWriter();
synchronized (sw.getBuffer()) {
return this.write(sw, indentFactor, 0).toString();
}
}
I do not understand the necessity for the synchronization here, as the
StringWriter is only local to the given method (and, why the
synchronization is on the Buffer). Is the synchronization really necessary
here, and if, why?