breaking up CSS rules across various, single focused files can help with making large or small changes to a site without causing conflicts due to order of load.
base is for the large sections of the page and deals primarily with background color and default sizes. This will help give the page a shape without adding functionality
layout can help lock down more of the grid structure by modifying the headers, footers and articles of a page. This organization helps keep a uniform shape across pages that can be easily ported around.
modules is for some sub-selectors and modifying how items a level down from the top elements will work. by keeping naming more specialized it can prevent conflicts later on when you expand out the number of same-type children
state is in place to over-write other styles for specific elements
theme is for a changing color, text, and other items across the page at the same time without affecting other layouts. Can change the visuals of a page without impact.
changing state houses the different states that an element can be in based on user interaction.