Majorly, it is used while coding threading related application. Background worker process ensures that one process should not get effected if another is running in a same form.

In Windows Forms (WinForms), background work generally refers to tasks or operations that run in the background, separate from the main UI thread. This approach keeps the user interface responsive while time-consuming tasks, such as data processing, file I/O, or network requests, execute in the background.

when we want to do time taking tasks in background such as reading/writing large text file, loading large data from database, etc and updating progress bar.

If you thing about background work so threading come into picture.

This is a in build control of Microsoft winforms. Basically it is used for progressbar related job.