Same as earlier versions. Different settings give you either -
many thumbnail pics of frames so you can locate frames in a clip (slowest) or
thumbnails at start and end of each clip so you can identuify different clips. (faster) or
no thumbnails-assumes you know what each clip contains or they are the same sceens anyway, (much faster)
During an editing session I am continually changing between them to speed up things
You generally only need the full thumbnails when zoomed in looking inside a single clip, use minimum or no thumbnails when zoomed out to skoot and down a project quickly.
The more thumbnails you want, the slower it takes to load them. (If you have a CIA speed computer you dont have to worry)
As Ted states, each has a plus, but most come with a minus, i.e. display re-draw of the thumbnails.
I'll normally set that display to First Frame, or to None, once I get into the real editing of the Project.
All is good to start, when one has longer Clips, with "scenes" in it. As the thumbnail display is just an emulation, one still probably needs to use the Program Monitor, and the Frame tick marks on the Time Ruler, to get to the exact Frame, where they would want to Cut. I will use the All thumbnails as a rough locator, and then the arrow cursor keys to jog to the exact Frame for my Cuts.
If an editor uses the Source Monitor to Trim their Clips with IN & OUT Points, and then just drag instances of that Clip to the Timeline, they probably will not rely on thumbnails at all, and the None, with just the Clip Name will be good enough.
As with many operations, PrE offers several ways to do things, depending on how the user likes to work, or to provide the workflow that suits them best.