These Ad Groups are within individual campaigns. The two ways that I use Ad Groups is to test keyword profitability and send keywords to different landing pages, but also to separate keywords by price.
If I am bidding $.10 per click and am not spending my daily or monthly budget, I might resume my expensive Ad Group that bids $.50 per click to increase the amount of my budget that is being spent. Then as my daily or monthly budget is being spent I can pause my expensive bidding on the expensive Ad Group.
This is just one of the many ways to use the Ad Groups, but it is great if you always want to be able to spend your budget.
