If you hear a combo/menu/set, you will consider as hamburger with french fries and coke. Therefore except at fast food restaurants, you will hardly see combo in Europe.
On the other hand, it is very normal to see combo on the menu list in Japan. Then, these combo are usually offered together with noodles; udon, soba or ramen.
Then, most popular combo is with ramen noodles. Usually the ramen noodle bars offer several combo. For example, ramen+gyoza dumplings, ramen+rice, ramen+fried rice, ramen+rice bowl. The concept of the combo is that the size of the noodle is the regular size but fried rice or rice bowl is smaller size. On the other hand, most of the restaurants also offer regular size fried rice and rice bowl with smaller size ramen noodle too.
These combo are suitable for two types of people. 1=Very hungry. One dish is not enough. 2=Cannot decide which dish to eat. OK, let's have both.