Lets take an example where we have to load a dyanmic set of files. Normally we would have to iterate through the array, grab one file, then the next, etc. PHP is a good example of this and it'd take it a really long time to access each file since the computer's disk is slower than the CPU. This is what we call I/O wait.
$files = array("file1.txt", "file2.txt")
for ($i = 0; $i < count($files); $i++) {
$fh = fopen($myFile, 'r');