I am not exactly sure what it is for, but know what it is, its a plastic cap plugged into a plastic camshaft cover, and not a very robust plastic cap at all.
Mine developed a small crack in it causing a P0471 code. Its not part of the PCV at all, but sure can screw it up with a massive air leak.
Made a wild guess, the designers of the 1.4L weren't sure which would be the front of the engine, so made provisions for two oil caps, but since they didn't need this extra hole, put a cheap plastic cap on it. Since my Cruze was under the PT warranty, took it in to get a new plastic cap, could have slapped some epoxy over that hole. But instead they replaced the entire camshaft cover with all new bolts. Just because of a cheap plastic cap that seems to serve no logical purpose! Except cause more problems.
But not the only part like this, was curious as to what would happen to my manual transmission if a one buck synchro would go bad, already had to replaced these before. Looks like with the Cruze, would have to replace the entire transmission for 1,800 bucks if one little part went wrong.
Never use to be this way, but sure is now. Can generally find a new camshaft cover kit on ebay for around 40 bucks, takes about a half an hour to replace it. Wonder if duct tape would work, that vacuum underneath it should hole it in place.