To the user, the advantage of multitasking is the ability to have several applications open and working at the same time. For example, a user can edit a file with one application while another application is recalculating a spreadsheet.
To the application developer, the advantage of multitasking is the ability to create applications that use more than one process and to create processes that use more than one thread of execution. For example, a process can have a user interface thread that manages interactions with the user (keyboard and mouse input), and worker threads that perform other tasks while the user interface thread waits for user input. If you give the user interface thread a higher priority, the application will be more responsive to the user, while the worker threads use the processor efficiently during the times when there is no user input.