The baselayer is your foundation for warmth, providing light insulation and wicking away perspiration.
In moderate temperatures or if you tend to run hot, a lightweight midlayer is the perfect, versatile option.
Are you always cold? Are you facing down frigid conditions? If you answered yes, an insulated midlayer is your MVP.
The outerlayer protects you from the elements. Choose a waterproof-breathable material for optimum happiness.