Stretching really depends on the fiber you're using. For example cotton has no memory, meaning it will stretch & stretch, while wool has excellent memory, so you can tug on it and it'll snap right back into place.
They're generally meant for weaving, but you can find them in just about any fiber type and use them however you'd like. They're usually a pretty good deal. One word of caution about using a WW cotton...your sweater will stretch like the dickens.
I can't believe I just used the word 'dickens' in a sentence...I'm getting so old.