No & means a photocopy: the function scribbles on its copy and you see nothing.

By default C++ passes each argument by value, so a function receives its own copy, even of a whole vector. Adding elements to that copy cannot reach the caller's vector, and copying a big container is slow too. A reference parameter makes the function work on the caller's own object. Remember that Java works differently for objects.

Read it once, then a question tests whether the trick stuck.