I believe it's to protect the prongs from getting damaged during shipping - and if the container gets rained on or something, it could provide some protection from that
I’m of the opinion that it’s just to improve the perception of package quality. I don’t see those cheap plastic pieces as keeping the prongs from accidentally warping or bending but I guess they could keep moisture away. I’ve never received a corroded set of prongs before their use though so my theory stands as they are upping the packaging visual aesthetics.
Could it be to protect the rest of the item from getting scratched by the prongs? Especially a screen, or some soft/glossy plastic case that would be prone to scratches.
I've been buying electrical and electronic things for years (okay decades), and the toys and computers I got as a kid never had these. The cords are always nicely coiled in their own cavity of the box too, so already protected well enough.
All in all it just seems like a solution to a non-existent problem.
It looks very easy to produce, what if it's for the illusion of higher quality? In which case the small increase in production cost is worth it for better sales (or allows them to sell the item at a higher price).
Same idea as the unnecessary packaging on fruits at the store
The plastic adds enough rigidity overall to help resist bending. The combination of the metal inside and the overall thickness of the added plastic is pretty good. It won't resist a Hulk smash or a tank rolling over it. But it should, hopefully, resist routine bending forces during packaging and shipping. Goof around with it the next time you receive one, and you'll see what I mean.