Synthetic insulation is possibly one of the most versatile pieces of garments you can own. A synthetic jacket is designed to be the alternative or in between a fleece jacket or a Down Jacket. The bonus of this type of garment is that it can get wet and will not cause any harm to the inner filling, whereas a down jacket when wet does not perform. It can be worn as an insulated inner layer beneath your waterproof jacket; It can be worn also over the top of the waterproof jacket when you stop for lunch as an insulated layer. Allot of climbers use this time of jacket as they perform better than a Down jacket or fleece when in alpine or mountain conditions.
What to look for: If you look at the higher specification jacket that use a Primaloft insulation, these jackets are designed to be used and a abused. The inner fibres are designed not to break down easily, especially when stuffing them and taking them out of a rucksack constantly. A cheaper non branded equivalent although will keep you as warm, will break down quicker, but is perfect for wearing as a general walker or keeping warm camping etc.