Groups are made up from multiple stores the number of which can be seen underneath the group name at the top of the table. Most groups to be compared with are unconsolidated meaning they are an average of their branches. Some groups are consolidated meaning they are a sum of their branches. Since no naming convention is forced it's up to your system to make it obvious which groups are consolidations and which aren't.
Comparing to groups can be very useful if you have relevant groups with lots of contributors. Usually, groups are divided two ways: Earnings and Location. Groups divided into earnings are useful for comparing your Financial data with directly. Groups divided by location are useful for comparing the areas where they are making money. Location groups are likely to have a similar distribution of sales and expenses, this can help you target the zones you need to work on to improve your business.
NB: Groups provide you with an accurate representation of the average for an account, from only the members that use that account. This means the totals for the parents may not appear to add up since they are the average value of that account, giving you a more accurate comparison.