Insulation is the thick wool like layer of fabric inside the sweatshirt. This is on the inside because it is supposed to be in contact with your skin. No matter how cold it is outside, your skin is usually warm and the insulation locks that warmth in to ensure you are always warm.
This coating should be thick or thin based on the requirement of your skin. You may need extra layer if you are easily cold and not so much if you are good to go. You can completely skip the insulation as well if your criteria is not warmth.
This layer has to be protected because it is soft and also needs extra care. Washing needs to be light and also regular. This is the most important layer so ensure that it is soft on your skin and thick enough to math your requirements. Ideally do invest in a nicely insulated sweatshirt because there can never be enough travel plans and the world is full of cold countries!