Why must there be " %s " to display the third argument

[menglingyu]

var err1 = new Error('No such file or directory');
var err2 = new VError(err1, 'failed to stat "%s"', '/junk');
var err3 = new VError(err2, 'request failed');
console.error(err3.message);  // request failed: failed to stat "/junk": No such file or directory

if  not "%s"
var err1 = new Error('No such file or directory');
var err2 = new VError(err1, 'failed to stat', '/junk');
var err3 = new VError(err2, 'request failed');

console.error(err3.message);  //request failed: failed to stat: No such file or directory