Background processes can roughly be divided into two categories: daemons and compute-intensive tasks.
Daemon processes offer services like web pages serving, email transferring, time synchronization, and similar. They usually consume little CPU and memory, and run quietly without user interaction. They mainly communicate with other computer programs, or with other computers via network.
The background is also used for long tasks that require huge amount of computation and thus CPU time. Running this kind of task at low priority may seem counterintuitive, but it becomes clearer when one considers that a computer typically spends more than 90% of its time waiting for user input. One can assign a high priority to interactive tasks, that will appear highly responsive, and leave the majority of the time to low priority tasks.
See also: Scheduling.