From what I got, the sentence Sie isst immer noch is translated to She's still eating.
I get what each word in the sentence translates to:
Sie -> She
isst -> eats/is eating
immer -> always
noch -> still
So what exactly is immer's role in the sentence there? Wouldn't Sie isst noch mean the same thing?
How does the expression immer noch differ from noch?
Sie isst immer noch.
Both immer and noch are used as modal particles in this sentence. I introduce the modal particle gerade to show the difference.
Sie isst. → She eats.
Sie isst gerade. → She is eating.
Sie isst noch. → She is still eating. (She's not ready to deal with you.)
Sie isst immer noch. → She is still eating. (Same as it was when you looked/asked last time.)
You may see immer noch as a modal particle of its own. It changes the mood to impatient.
Steht ihr im Stau? → Are you stuck in a traffic jam? (curious)
Steht ihr noch im Stau? → Are you still stuck in a traffic jam? (commiserate)
Steht ihr immer noch im Stau? → Are you still stuck in a traffic jam? (impatient)
"noch" means that the eating is ongoing, but focuses on the fact that she'll be stopping at some future point in time.
"immer noch" means that the eating is ongoing and has done so for a long time.
Compare:
Das neue System ist noch nicht hochgefahren. (Presumably it will soon be.)
Das alte System ist immer noch nicht ersetzt. ("We've been waiting for years! What's taking them so long??")
