You’ll need to consider a hood with a higher CFM if you:
frequently do a lot of frying or wok style cooking
grill on your range top (indoors or outdoors)
cook ethnic, spicy, or fishy foods
mount your hood outdoors
have a range top with very high BTUs
have an extra long run of ducting (especially with 90-degree bends)
position your hood especially high over the range top (over 36 inches)
want to keep the noise level down by choosing an extra powerful hood but running it at a lower setting