In Chinese culture, food is central to social life, hospitality, and celebration. Sharing a meal is a primary way to build relationships (关系, guānxi). Therefore, knowing how to properly compliment the food is an important social grace. While a foreigner saying “好吃 (hǎochī)!” is always appreciated, using 可口 (kěkǒu) can demonstrate a slightly higher level of linguistic sophistication. It's a more measured and sometimes more refined compliment. Think of the difference in English between “This is yummy!” and “This is very palatable.”
The most common point of confusion for learners is the difference between 可口, 好吃, and 好喝.