It's hard for people to make the comparisons between brands, sizes, so on and so forth. Australia recently passed legislation requiring supermarkets to list the price-per-unit-weight/volume alongside the product price. So on a food item, you'll see the price for the product, and also the price per 100 grams.
It means things are directly comparable across brands and sizes ("how does this 117g portion compare to this 270g one?"). It's a fantastic bit of legislation.
Britain also has this kind of legislation. But e.g. Tesco often tries to play around it, by listing, say, different kinds of apples sometimes by piece, sometimes by kg.
I'm not sure it's a law though (I doubt it, at least it's not federal, as it's not in all stores). I imagine it could just as easily have been market pressure, as the supermarkets began to offer their own brands of products for cheaper than premium, it made sense to offer direct comparisons with as much information as possible.
It means things are directly comparable across brands and sizes ("how does this 117g portion compare to this 270g one?"). It's a fantastic bit of legislation.