To find where a products is being sold for the cheapest price.
For electronics I know 3rd party services that compare prices for different shops. But in cases like "Where can I by this specific dental floss" I sometimes use google shopping search.
I find it super useful for listing all the places in my country where I can buy X, orders by price. Doing a more generic search for a product tends to give results heavily dominated by sites in the US. Also really useful when combined with image search. I can take a picture of a cool lamp and quickly find all the places where I can buy a similar lamp.
What is it for?