The actual structure of the Linux (or Unix) filesystem, that is the names and layout of the directories in the filesystem, appear to be random and complex. In reality the historical usage and structure has been somewhat random in the past, depending at least in part upon the standards and practices of individual organizations and the specific lineage of the Unix or Linux version in use.
The Importance of Standardization
The primary reason to standardize the directory structure and usage of the Linux filesystem is to make life easier for the system administrator. Knowing where to look for files by usage or type makes one’s life much easier. This is especially true when many different Linux distributions can be running side by side in the same organization. Consistency makes it easier for system administrators to switch between distributions when working.
Filesystem Hierarchy Standard
There is a Filesystem Hierarchy Standard that specifies required and optional directories and the uses for each. Although this standard is still not strictly adhered to by all Linux distributions, most are currently in the process of converging their implementation to meet the standard.
You can use the link, above, to view the entire hierarchy on the web, or you can use this link to locate and download a PDF file of the standard.
The standard is very loose and so you will probably always see some differences between the way different distributions interpret and implement the standard.