I mean effectually - I understand the functional difference. What different GAIN do I receive from Arnold presses?
The two exercises are similar enough that you can use them interchangeably if you so desired. Personally, I stick to the standard press and only occasionally use the arnold press to break up the redundancy; even then, I am considering taking arnold press totally out of the lineup because of the strain it puts on my rotators (your mileage may vary).
The way I feel it is that Arnold presses take more time per rep, meaning that your front shoulders feel more stress during the movement as it takes longer.
Perhaps your middle shoulder parts (outer parts) benefit more from arnold presses as you have this twisting motion when lifting the weights. You don't have that when doing standard dumbbell press.