Question regarding Webmaster Tools

Alright, I know I've been a long-time vBSEO customer but there's a silly question that gotta be asked regarding Google's Webmaster Tools. The question is, do I have to resubmit my sitemap daily manually by logging into the Webmaster Tools and click on the "Resubmit Selected" button or it does that automatically? Seriously, I don't know what's the deal with the "Last Downloaded" column and what does it mean.

No, there is no need to resubmit it daily. vBSEO Sitemap Generator will ping Google automatically every time new sitemap is generated.

But why does the "Submitted" column show up as, let's say that I've resubmitted the sitemap last night (March 9th) and it says "Submitted: Mar 7, 2008" (just an example). Shouldn't it resubmit the sitemap automatically too and update its date when it was last generated?