Searing will create a Maillard reaction caused by heating proteins and carbohydrates. It leads to browning and new flavors. It is different from caramelization since caramelization involves only carbohydrates. One of the secrets of this recipe is the searing. You get a nice Maillard reaction (the searing/browning) to add lots of flavors you get no other way. Get them close to the final color you like during the searing before you move on to the oven-roasting step. Just baking them in the oven will not get you there/