Job attributes determine how the system runs each job. Some job attributes are set from the user profile. Other job attributes come from system values, from locales, from a Submit Job (SBMJOB) command, from a job description, and from the Change Job (CHGJOB) command (from which you can change values for attributes while the job is running).
Controlling job attributes gives you the flexibility to control jobs at the job level, user level, or system level. For example, you can have your system set up to go all the way to the system value to get job attributes (which is the system default). Then if you want to change a value for all new jobs on the system, you can change the system value.
By specifying a value in a job description, you can affect all of the types of jobs that use that job description. For example, if all of your batch jobs use the same job description, then changing the job description for the batch jobs can affect all of your batch jobs and leave all other jobs unaffected.