On a longer journey, I can't see that it will make much difference whether you give EV priority or not, since you'll have exhausted the battery (and the ICE will therefore start) long before journey's end. But you should find it very useful on shorter trips and if you ever visit a city with an 'eco' zone where you must use electric only (I'm not aware that there are any in the UK, but there are a few on the continent).
By default, the car will let the battery deplete to 'empty' (which is not really zero charge) and then maintain that state by charging from the petrol engine. You may notice (though it's hard to hear it in normal driving) the engine starting and then stopping to allow the car to drive on electric for a while, and then the engine starting again as the charge level drops. In this state, it's behaving very much like a standard (non plug-in) hybrid, such as a Prius.
If you press the 'Save' button it will retain the battery's charge level from that point on. 'Charge' forces the petrol engine to recharge the battery up to ~80% 'full' (getting those last few percent from the engine isn't very efficient, 'fast' chargers available at service stations etc also stop at about the same level) - but the only time you'd want to do that is if you're towing and approaching a very long, steep incline where you need close to maximum power from the battery and the engine, with a danger of depleting the battery and dropping into 'tortoise' mode.
As I said initially, this is all fascinating stuff (IMNVHO), but there's absolutely nothing wrong with just putting it into Drive and letting the car sort it out for itself (while you enjoy the quiet driving experience without worrying yourself about button pressing).