Depending on the country in which you are, giving that extra money to the waiter for his services can be considered in certain countries as an obligation or even be taken as an offense.
In countries like the United States and Canada it is an obligation to pay the tip and the percentage usually ranges between 15 and 20%, since most of the waiters' salary comes from tips.
In contrast, in other countries such as China or Japan it is considered as an offense and it is frowned upon. Except the Chinese regions of Macao and Hong Kong.
In the case of Spain, as in Portugal and Italy, rounding is usually given as a tip.
In other countries, such as the United Kingdom, it is usually included on the ticket with the term “service included”.
So, if you go to a local restaurant and give the waiter a considerable tip, he will be eternally grateful.