features: 
what does it do? 
In the old settings view, under the content management you can accept cookies for the duration of using the browser then the cookies are deleted. I'm not sure about the usefulness of that in actuality.. however, I appreciate having that setting and I always use it.

why do we need it?
It seems mildly more secure.. in that if someone wants to use my computer, then I just close the browser and re-open it and now I know that no website it going to auto log me in. 

bugs: repro steps (I did X and expected Y but Z happened!)
I'm using the latest available download for google chrome canary. When I go to the (new) settings page -> advanced -> site settings -> cookies.. I expect to have a setting that is called "keep local data only until you quite your browser". However, I do not have that setting.