Jobs are what your crews will track their time against. They make every minute of payroll time accountable to something you can track. Then you know where all your hours are getting spent - each day, each week, and even the total for the year.
Jobs can be customer jobsites, but they can also be jobs to track internal/unbillable time, like Drivetime or Shop (Mechanical). Jobs can also be imported from estimates, or created from scratch.