As per MI....

"We do not include the longer port door rod because not all 308 uppers require it. If we sent the longer rod with every 308 hand guard then the customer would think it was mandatory, and we would get complaints of how the barrel nut hit/bent the rod and damaged the upper. The other half of customers would complain we charged the $5 for something they didn't need and can they get a return label so they can mail it back for a refund. In the wide world of 308's there is no consistency, so if you ever have questions please let us know and we can try to help out."

Basically some setups using the MI handguards needed the longer rod , because of the type of MI handguard attachment , to help retain the ejection port door rod.

A OEM ( shorter one )  one could slip out of the rear holder

I ended up buying some , because I didn't want to find out after a mock up assm. that I needed it . My PSA's needed it.

You can just make out in the photo above , how the barrel nut is a little short of contact with the upper , flush wise.

The extended rod takes up the slack.