There are 3 ways to schedule in BACKGROUND:
SM36
SE38
SA38
The easiest of the three is SA38.
In foreground JOBS are only allowed a CERTAIN amount of runtime. Long running jobs usually times out in foreground, and have to be RUN background. Some customers has day-end jobs to fill custom tables, and these only run late at night, so they are scheduled as background jobs as well. There MAY be any of a hundred reasons why you want a job to run in background instead of foreground, and these are only 2 of them.

There are 3 ways to schedule in background:
SM36
SE38
SA38
The easiest of the three is SA38.
In foreground jobs are only allowed a certain amount of runtime. Long running jobs usually times out in foreground, and have to be run background. Some customers has day-end jobs to fill custom tables, and these only run late at night, so they are scheduled as background jobs as well. There may be any of a hundred reasons why you want a job to run in background instead of foreground, and these are only 2 of them.