A good lining is important for a jacket or blazer for several reasons:

Comfort: The lining helps to prevent the outer fabric of the jacket or blazer from rubbing against the skin, which can cause irritation and discomfort. The lining can also provide an additional layer of insulation, keeping the wearer warm and comfortable. 

Durability: A good lining can help to prolong the life of a jacket or blazer by protecting the outer fabric from wear and tear. The lining can also prevent the fabric from stretching or losing its shape over time. 

Appearance: The lining can improve the appearance of the jacket or blazer by providing a smooth, polished look. It can also help the jacket or blazer to drape properly and maintain its shape. 

Functionality: The lining can add functionality to the jacket or blazer by providing pockets for storage or concealing seams and stitching.

Maintenance: A good lining can make the jacket or blazer easier to maintain by protecting the fabric from sweat, oil, and other stains. It can also be easier to clean and care for than the outer fabric of the jacket or blazer.