Indeed, "why" in some culture tipping is more important while in others it is not could lead to very interesting historical/cultural considerations.

So, what are the reasons why tipping in the US is considered important and why the amount is such? I just assumed it is a tradition which simply resisted to time changes, since when waitstaff were in fact living only/mostly on tips from customers, and were not regulated by a more formalized and structured labor law and "real" salary. And, my assumption was such because that's what happened here as well. For some reason, the idea that waitstaff was in fact regularly employed by restaurant owners (with salary and benefits) made its way into people perceptions here and tipping slowly decreased in importance and amount. Today, all it really means is: leave anything as a sign you're happy, but if you don't it is still perfectly ok". That's it.

On the other hand, many people, including me actually, tend to tip, or tip more, if they are particularly happy. I am personally all in favor of positive reinforcements, and if I think a waiter was particularly good and careful, I tend to tip significantly more.

I am also thinking that tipping here is considered appropriate in more formal situations. For example, if I eat in a restaurant for my usual 1-hr lunch break fromt he office, tipping is really not expected (but always welcome, of course), because it is not considered a "formal" lunch. On the contrary, usually it is a restaurant here that may give you a little discount (!!). Probably this is a consequence of the competition standard restaurants had to face against fast-food ones, or bars or standing pizza-places, or other types of places where you don't actually sit to a table to eat - which tend to be significantly cheaper.