Also not sure why you'd go through the trouble of forcing p0 when you could just OC the memory right out of the gate. Less messing around. Maybe makes your OCD feel better? It doesn't seem to increase voltage to the memory and it doesn't allow you to OC the memory further.
Because there's a bug (or is that a feature?) in nvidia drivers or mining software that makes the memory clock jump ~ 200 MHz up when you close the miner's window. I didn't spend too much time investigating it, so maybe it's not all driver versions and/or not all miners that do that, but every single one I tried behaved that way. Let's say you've got gtx 1080 and your stock memory clock is supposed to be 5000. When you set OC to +400 (in MSI AB, for example), and run the miner, your actual memclock will be ~ 5200 or smth like that, and not 5400 (because cuda apps run in p2 instead of p0). Which would be ok, you could just add +600 instead to reach the clock you wanted (5400)... except when you close the miner, the memclock will jump to 5600 for a brief moment. And that is a problem - if you're pushing you memory overclock close to its max, then you'll get the driver crash in that short moment when the clock jumps to +200 more.
If you force cuda apps to run in p0 mode, then that jump is gone, the memory clock stays at whatever you set it to even when you close the miner window. And yeah, it does allow you to OC memory further, because without that weird jump you don't have to leave so much headroom for that brief clock increase and can set higher memclock.