When we think we often follow several trains of thought at once. When we search online there are many times when I have two trains of searching.
The top tab bar is one train of thought. The bottom is the second. It is like having two separate Safari or Firefox window, but they are both up at once. You can split them apart if you want.
Also - the ability to save chains of tabs. This would be handy for those searches that you never quite finish or that you have to go back to. (Podcasts or instructional videos - one leads to another, Equipment reviews - one leads to another, you get the point)