Yet another ?, this one on plastic

[Plaz]

What is the purpose of putting plastic on top of tomato containers. I've read where the earth boxes come with it and the GS boxes (if bought with a kit) do as well. I bought only the GS boxes and wonder if I need to add plastic? Or add mulch? Or a combo of both. I started putting plastic on them but it was such a pain in the butt that I decided against it.

Thanks!