In many countries in Asia, it's considered rude to hand money with a single hand. You supposed to use both your hands like that:
It's most common (among places where I've been) in China and South Korea.
There's also less official method dominating in Vietnam and Sri Lanka (all of the sudden):
Once I learned this thing I started using it without a second thought. It felt natural, and whenever I notice a "rude" foreigner using one hand, I would give my self an imaginary pat on the back. Today I decided to share it and realized that I have no idea about the origin of the gesture.
I turned out that I do not alone take it as granted and there are tons of resources teaching etiquette but just a few trying to explain the phenomenon.
The best guess that I found is that it shows your mindfulness towards the recipient and that the gesture is intentional.
When you think about that it kind of makes sense and also explains why in South Korea restaurants you never leave money on the table. Once we did that and the waiter screaming MONEY went chasing us to the street.
Please, share your opinion in the comments.