I quite often find myself having multiple tab groups for a single category but that would be best for organisation for them to stay in their own tab groups.
I would like to be able to move tab groups in to tab groups so that I can organise my tabs better.
An example of this would be researching say the Olympics and their sports. You would have the main group Olympics which would contain tabs for general information like history and tickets, and also the tab groups for the individual sports so you could have tabs for the sports, athletes and such.
I'm sure that you can see that having everything in 1 tab group for the Olympics would get very large and hard to find the tabs you need. But by allowing tab groups to be placed in other tab groups you can stay organised.