Because I know that every corporate app dev is (NOT) going to write:

   StringBuilder buf = new StringBuilder( s1);

   buf.append( s2);

   String cat = buf.toString();

instead of:

   String cat = s1 + s1;

in their application code. (although I think that is pretty much what the compiler generates on your behalf, anyway)

The loop is in the benchmark so I don't have to write code dealing with many different string values of differing size manually.