Do you supposed to use it?

Mills

I don't see why not. Extract beers have hops and yeast in 'em, too, right?

Just found it. Page 281 How to Brew:

"A good clarifier for almost all worts though not recommended for high adjunct or extract based worts."

Mills

Shows what I know.....

Mills

A whirfloc tablet comes in every extract kit I get from Morebeer and their instructions indicate that you should dump it in with 5 min left on the boil. When making extract beer you definitely get trub. The whirfloc helps knock this down and leave it in your boil pot (I assume that is the point of it).

I just did a brew a few weeks ago and forgot the tablet. It's in the primary and definitely not clear. I'll have to hit it with jello and hope that it cleans it up as much as I like (though it is for personal consumption so really I don't mind if it is cloudy).

The problem is extract has already gone through the boil process and the hot and cold break material has already been removed...that is what whirlfloc is used for....to coagulate those two things so they can be separated...nothing else...

If it's an extract with specialty grains, you can still get grain gunk that it can help with.