Why to use StringBuffer in Dart instead of Iterable.join?

There isn't a big difference. If you already have a list of strings, there is no difference in using StringBuffer.writeAll or Iterable.join. The Iterable.join method uses a StringBuffer internaly:

String join([String separator = ""]) {
 StringBuffer buffer = new StringBuffer();
 buffer.writeAll(this, separator);
 return buffer.toString();
}

From the Dart documentation (click on the code button on the right).