System Administrator's duties

The Linux operating system provides for two classes of users :

1. normal users
2. superusers 
3. software accounts

The term superuser is commonly used in many operating systems although in Linux, we call such a user root. The root user (or users) has access to all system commands and so can access all system resources through those commands.
Normal users have greatly restricted access in that they can execute public programs, access public files and access their own file space.

The role of root is to properly administer the computer system. Operating systems divide accessibility into two or more categories, the ordinary user and the administrator (sometimes called privileged mode). Some operating systems have intermediate categories between the two extremes where a user is given more privileges(e.g. through the sudo command) but not full administrator privileges.
The reason for the division between normal user and administrator modes is to ensure that normal users can not impact other users.
In a work environment, keeping data secure becomes even more important. Different users would have access to different types of data (financial, personnel, management, research), based on their identified role within the organization. Performing administrator duties (creating accounts, scheduling backups, installing software, etc.), if performed by the wrong person, can cause disastrous results if the person does not do things correctly. E.g. imagine that when installing new software, the person unknowingly wipes out the boot sector of the disk. Upon reboot, the computer no longer functions correctly.
And so we have a unique account in all operating systems that is capable of full system access. It is through this account that all (or most) administrative functions will be performed.

What does a system administrator do? The role of the administrator will vary based on the number of users of the computer system(s), the complexity of the computer system(s), the types of software made available, and more significantly, the size of the organization.

A small organization with just a few employees might employ a single system administrator who is also in charge of network administration, computer security, and user training.
In a large organization, there may be several system administrators, several network administrators, a few people specifically in charge of all aspects of security, and another group in charge of training.

The following list is common to many system administrators:

• Install the operating system
• Update the operating system when needed
• Configure the operating system to fit the needs of the users in the organization
• Secure the operating system
• Configure and maintain network communication
• Install, configure, and maintain application software
• Create and manage user accounts and ensure the use of strong passwords
• Install and troubleshoot hardware connected to computers directly or through a network
• Manage the file system including partitioning the disk drives and performing backups
• Schedule operations as needed such as backing up file systems, mounting and unmounting file systems, updating the operating system and other application software, examining log files for troubleshooting and suspicious activity
• Define (for your organization’s management) computer usage policies and disaster recovery plans
• Create documentation and training materials for users
• Make recommendations for system upgrades to management

System administrators may not be responsible for all of the above duties. Other forms of administration (e.g., network administration, webserver administration, database administration, DNS administration, and computer security specialist) may take on some of the duties or have overlapping duties with the system administrator(s).
For instance, a network administrator would be in charge of installing, configuring, and securing the network but the system administrator may also be involved by configuring each individual workstation to the network. A webserver administrator would be in charge of configuring, maintaining, and troubleshooting the webserver but the system administrator may be in charge of installing it and setting up a special account for the webserver administrator so that he/she can access some system files.

Resources

Linux with Operating System Concepts by Richard Fox
isbn:9781482235906, goodreads:20792170