Double quotes

Whether typed or "air quotes", quoting things has become a cultural practice to indicate something...but what exactly does it indicate? I love Chris Farley's SNL sketch in which he uses air quotes to explain that the mainstream media doesn't like him because he isn't "the norm" and isn't "camera-friendly"...because he doesn't "bathe regularly" and because he "scares small children"...

https://www.youtube.com/watch?v=s_cAS-mvV20

We always seem to use this gesture when speaking to mean "the thing in quotes isn't to be take literally"...as a signal that there's a double entendre. As if you're saying "the official party line is 'whatever's in quotes' but don't believe it". For example, Joe and Fred are at a baseball game on a Tuesday afternoon.

Joe: Hey man, I thought you had to work today.
Fred: I did, but I'm taking a "sick day"

In print it's a little different. Obviously, the traditional use of quotes is identify text/speech not of the author writing the text. Then travelling around I noticed that in some regions (seems like the South), they are used for emphasis in the say way we might bold, italicize, or underline text. So you might see a sign that says

Come Monday for "Free Tomatoes"

Is there a catch? Are the tomatoes not really free?

Come Monday for "Free" Tomatoes

Are they not really tomatoes?

Come Monday for Free "Tomatoes"

I have no idea where this comes from, but it can make for some pretty funny signs given how we used the air quote gesture in conversation.