Large spaces need to be divided into regions with edges creating perceivable differences between them, so people can understand where they are within the larger space and what rules and expectations apply there. When people cross edges into different regions, they should be able to tell that they’ve done so.
In the real world, neighborhoods help people make sense of a city by breaking it down into manageable chunks. These neighborhoods are best if they look a certain way that’s different from the surrounding neighborhoods, so people can immediately tell where they are. In grocery stores, flooring, shelving, ceilings, or other architectural features indicate what the different sections are and what kinds of goods you can expect there.
In the digital world, designers create edges by making two sections look different, whether they’re regions of the same page, or regions of the site. Designers usually give the footer and/or the navigation a different background color from the main body of the page, to indicate that it works differently. Similarly, a marketing section and a support section might have a different visual character, to indicate that one is aspirational and the other is utilitarian.