I have explained the significance of business group in very much detail in my videos.
The HR data, e.g. employees information, is segregated by business groups.
In short,
If you have one big list of employees shared among operating units then create one business group.
If the operating units have their own employees and you do not want OU1 to see the list of employees belonging to OU2, in that case you can create one business group per operating units.
These are two extreme examples. There could be hybrid cases in between them as well. Lets say two operating units sharing the employees and the other two having their own employees.