Why Group Crashes?
A crash group lets you organize a set of call stacks using some level of the call stack hierarchy other than the top-most stack frame. Imagine you have the following three call stacks. By default, these would all be grouped using the top-most stack frame (stack key A), and you would have just one entry in the Summary page that grouped all three crashes.
A->B->C A->D->E A->F->G
If you set up a crash group at level 2, stack key A would no longer appear in the summary table. Instead, you would have three different groups displayed (B, D, and F).