Greetings!
So, my AR magazines have been coming with what they are calling dust covers. It’s a little piece of plastic that sits on top of the magazine that you have to take off to load it into your rifle.
What is the purpose of this thingamabob? Is it important, or is it just a dohickey there to make you feel like you got something extra with your purchase?
If you were playing in the sand box with your mags they would be important, otherwise there just a doohickey to make you feel good. 
My observation is it compresses the loaded rounds down just a little bit to take long term storage stress off the plastic feed lips. I don’t remember reading this anywhere, just my observation.
Does the dohickey come with metal magazines? I don’t have any metal magazines so just wondering.
Edit: This is what Magpul has to say about it on their website:
" In addition, an included pop-off impact / dust cover can optionally be used to minimize debris intrusion and protect against potential damage to the top cartridge during storage and transit."