I have numerous Lock & Lock containers and find them invaluable for airtight storage. Unlike many other plastic containers they are easy to open when they've been in the freezer: very useful for things you want to store frozen but repeatedly remove small quantities of, for example, frozen nuts (they keep much longer if frozen); or sprigs of herbs in this size or things like peas, coffee beans in larger containers. This 350ml size is perfect for 100g packs of nuts and most 250g packs of butter fit snugly. I actually freeze quite a bit of butter as I like a particular organic brand of fresh butter that doesn't keep as long as others in the fridge. In the fridge this 350ml box stops your butter picking up any strong flavours that might taint it: butter is very vulnerable to picking up taints.