Actually, this method can be useful to test a generator before iterating, as it executes your function up to the first yield statement. I.e. if you try to read a non-existent file in a generator, an error will normally occur only in client code foreach()'s first iteration. Sometimes this can be critical to check beforehand.Take a look at a modified example from here:http://php.net/manual/ru/language.generators.overview.php#112985<?phpfunction getLines($file) { $f = fopen($file, 'r'); try { while ($line = fgets($f)) { yield $line; } } finally { fclose($f); }}$getLines = getLines('no_such_file.txt');$getLines->rewind(); // with ->rewind(), a file read error will be thrown here and a log file will not be clearedopenAndClearLogFile();foreach ($getLines as $n => $line) { // without ->rewind(), the script will die here and your log file will be cleared writeToLogFile('reading: ' . $line . "\n");}closeLogFile();?>P.S.: When you iterate over a generator after ->rewind(), you'll get the first yielded value immediately, as the preceding code was already executed.