In cooking, it is important to have the same sizes of the ingredients to have the same time of the doneness of cooking. By using this, it will help saving the time to slice the foods properly and to have a uniform thickness. Also, this utensil is very useful in kitchen to make different cuts in easier way. There are different blades that can be attached to the mandoline to create Julienne cuts, wavy cuts, and crinkle cuts.
Sources:
http://thehairpin.com/2012/11/the-history-of-the-mandolin/
http://chefharvey.com/mandoline.html