With P and E cores, all the Blender stuff can be offloaded to the P core, all the background stuff (OS, AV etc.) can be stuck onto the E cores, this frees up the P cores to use 100% of their CPU instead of the 90+% that is only available because of background tasks
Or you could just have more P-cores so that your background tasks are using 1 P-core while the rendering is using 15 P-cores for example. That'd give better rendering performance than having 8 P-cores for rendering and 8 E-cores for the background tasks.
No, I don't think that's the advantage. The advantage is when you aren't using the computer for high-CPU tasks, you can have all or most of the inefficient (but fast) P-cores shutdown/idle, while just using the effiient (slow but fast enough) E-cores. So if you are just sitting there reading el Reg and not much else, you can be doing all that on the E-cores and let the P-cores sleep. Very little modern desktop tasks require a lot of CPU for anything outside short infrequent bursts - browsing, word processing, emailing, social-media'ing, etc.
Tasks that do require CPU horsepower tend to be things like games, rendering and other media 'modification'-related (e.g. editing, trans/encoding, etc.) tasks, other 'specialist' tasks that the average person wouldn't be doing. Which, on average across all desktops sold, is a tiny percentage of desktop computer usage on average across the entire population's computer hours. Sure, I might use a lot of CPU for long periods of time playing games or transcoding videos, but my parents don't, my sister and brother don't, most of my neighbours don't, my 2 work desktop computers don't.