Why does there need to be a planar intermediate in 2,2'-dibromobiphenyl when one of the phenyl groups rotates?
I think that your professor was just mentioning a rotational angle vs relative energy plot. It is really important to understand relative energy difference between different points in such plots. The situation when two bromines are in the same plane and close to each other will represent the highest energy spot, this actually means that the probability of the molecule to be in such spot is extremely low, but technically not zero, that is why some reaction that require such spot may happen. In the picture below you can see the plot for biphenyl, in case of the dibromobiphenyl one of the peaks will be much higher then the others. But you are actually right, molecule itself would prefer to rotate the other way.