Some cookie recipes, like this one, state not to overmix the dough when combining the ingredients. The biggest reason for this is that it can make the dough too mixed, causing the final cookie products to be hard and have a different texture. Overmixing the dough can also cause air pockets to get into the dough, creating a consistency throughout the cookie that is different from a soft and tender texture. To just “combine” ingredients, as often mentioned, is to stir until, visually, the ingredients are all mixed well together. There is no more mixing needed beyond that.