It's just that by filing, notice is given to the world, so to speak. If you ever have some spare time on your hands make a visit to the Chancery Clerk's office' and do some browsing. It is a an interesting experience. Most developers have a concept in mind when they build a subdivision - a way they envision what the subdivision will be like. In order to make certain that things don't get out of hand, they place covenants on the property and then file these covenants in the Chancery Clerk's Office along with the plan of the subdivision.
Covenants come in many flavors, depending on the preferences of the developer.
For example, one fairly common example of a covenant would be minimum house sizes. The minimum size is usually expressed in square footage. The purpose of a minimum house size requirement is to maintain the integrity of the subdivision. Imagine a subdivision with houses ranging in size from 950 square feet to 3,500 square feet. The value of the properties would be less that if all the houses were in the same size range.