It may be a relatively old fashioned concept nowadays but, if you are in to fresh food and making the most of home grown produce, you simply need the space to store jam, marmalade, pickled fruit and vegetables, etc. The same is true if you buy food in bulk, such as sacks of rice. Modern kitchens simply don't have the required storage space for stuff like this. We grew up in homes with larders and have rented houses with them too. They are an essential accessory to any kitchen, if you have the space for one.
The larder will be vented to the outside on the north side of the property, to maintain a naturally cool environment in which to store food. It will also be well insulated.
There are regulations regarding the angle of stairs that have to be met in new builds.