What is the meaning of premix cakes?

By the 1930s, consumers were buying enough bread and pastries from bakeries and grocery stores that home sales of flour was falling. The flour industry began in that decade to premix ingredients for sale as cake mixes, to which the cook needed to add only water.

The dry ingredients used for some baked goods, especially cakes, are premixed industrially and sold as prepared mixes, and while the user must add the fluid ingredients and bake the products, it is still a convenient system for the consumer.

In premixes, the flour, egg powder, shortening, fruit or flavoring components, and leavening agents are combined in the dry state, although, when mixed, the shortening is in the melted or liquid state and emulsifiers, such as monoglycerides, which improve air incorporation during mixing, may be used.

Some cake premixes sold in Britain contain, in powder form, all the ingredients required for a cake, viz. flour, fat, sugar, baking powder, milk powder, eggs, flavoring and color and need only the addition of water before baking.

Most nutrients, including all of the iron sources, are very stable in such premix. The premix can be kept for a year or more before being used as long as it is kept sealed and not exposed to high temperatures or humidity.
What is the meaning of premix cakes?