Combo meal is not common in European restaurants except fast food chains as like McDonald's, KFC and Burger King. If you order spaghetti in Rome, you will not get anything together with the spaghetti. If you order Wiener schnitzel in Vienna, you will get potato salad with it but hardly get anything else than salad.
Combo meal is very popular and normal in Japan, especially at Chinese restaurants. This is good choice for customers who are quite hungry or want to eat more. Instead of ordering two main dishes, you can have the combo meal which will be more volume than the normal meal. Then, the prices are very reasonable. If the ramen noodle is JPY 500 and gyoza dumplings are JPY 250, the combo will be cheaper than JPY 750. The price must be cheaper to attract the customer to order the combo meal and pay more than the single ramen noodle.
Typical combo meal is ramen noodle with fried rice(half portion) or ramen noodles with gyoza dumplings - the amount of the gyoza can be less than the single item(for example, normal gyoza can be 6 pieces but for combo can be just 3 pieces). Also many restaurants offer several types of combo meal. Usually they are called A set, B set or C set. If you see a word set, it means combo meal. Then,. please note that except at fast food chains, usually beverage is not included in the combo meals.