Imagine going to a new city, renting a car, and driving to a new location without a GPS or access to Google Maps. You would be lost without a landmark or even a general understanding of the neighborhood. This experience is similar for the FileMaker developer who takes over a solution from another developer. In the typical situation, you have to follow the trail of breadcrumbs from the original developer.
These breadcrumbs, or development standards, organize the code so someone else can follow your work. The better the organization, the easier it is for the developer (yourself or someone else) to come back later and get oriented with the solution. The code is simply the elements in the FileMaker database: tables, fields, relationships, layouts, scripts, etc.