With a lining that does not close off the entire inside of your jacket, the heat will not stay trapped inside. ... Less lining means less constriction of air flow. A half-lined jacket allows the fabric to breathe so that air can pass through the cloth and your body stays cooler. Less layers makes the jacket lighter weight.

What is a half-lined jacket?

Partially Lined Jackets

Half-lined jackets usually have lining across the upper half of the back and on the sides of the jacket interior, but not on the lower half. Quarter-lined jackets are very similar but without the lining on the sides of the jacket interior.

Why are some suits unlined?

Unlined - Unstructured Jackets

It gives the wearer the most natural look and freedom of personal expression. By removing the interior lining, shoulder pads, and the canvassing, the jacket wears more like a shirt than a suit coat.