Generally speaking, serrated knives are used to cut soft products that have a hard crust, such as bread and tomato.
Here is a list of food that are best cut with a serrated knife.
Tomato – You probably have tried to cut tender tomatoes without using a serrated knife and got splashed by tomato juice (I did!). Serrated knives can cut through tender tomatoes into thin pieces. They also work well on fruits that have similar thick skin / tender flesh such as peaches and nectarines.
Chocolate – If you want to break chocolate into small pieces for cooking, a serrated knife is your best friend.
Melon – A serrated knife is perfect to shave away the outer rind.
Cake – You have probably seen this many times… especially at a wedding when the bride and groom are cutting a wedding cake. Yes, they use a serrated knife.
Bread loaves – well, this is why some people call a serrated knife as a “bread knife”.