To be fair, most enthusiasts would know the difference, and it’s intended use. But to newer gun owners, I can see how it is misleading.
The plug is for a the DFCO Kung Fu grip, I believe.
This is just an incentive, to get you to buy the DFCO Kung Fu grip too.
Because without it, the plug is useless.
I have both on a budget Aero/Geissele build, and it stays tight, and serves its purpose.
Hope that helps.