So what exactly is Job Group? 

In this System, what Job Group meant is similar to the word 'category' in layman term. It is used to categorized different group of people, job function, or even location. Or you can also imagine it as a tagging system to differentiate different groups. 

Why do we need to have a Job Group? 

Job Group is meant to help User to perform their HR related task easily. For any organization, there will be different function, job and department to be conducted by Employee. It is important to categorize them in different category (or 'Job Group' as defined by the system), such as using Department of Marketing to refer to group of people performing similar task, or Senior Manager to refer to Employee with certain level of experience and ability, et cetara. 

All of the above (Department, Senior Manager, etc) can be set up in the system by creating Job Group. You can create a Job Group of 'Department', then under this Job Group, you can create different type of Departments, such as Marketing, Finance, Sales, et cetera.