Linux is a multi-tasking, multi-user operating system, so it can easily accommodate many users' needs. Even if your machine isn't attached to a network, you can share your machine with others -- each of whom will have their own settings.
Because of user accounts, Linux makes it easy to share your machine while preserving customized settings. Using accounts is convenient for users; it's also secure, because ordinary users don't have permission to damage or delete sensitive system data. Only one account, the root account can make changes to the system -- and to others' account information.
Using your system's root account for everyday work can be dangerous, because you can easily damage your system by accidentally deleting or modifying a sensitive file.
Most tasks in the following chapters don't require that you be logged in to your system's root account. Instead, you can accomplish almost everything from your user account -- so you won't accidentally harm your system.
Unless you're specifically told that you should be logged in as root, log in to your user account to accomplish tasks in this manual -- without risking disaster.