Although synthetic insulation sleeping bag is less thermal performance than down sleeping bag, It makes up for the shortcomings of the down sleeping bag that can't be used in a wet environment.
Synthetic insulation is typically made of polyester, It’s quick-drying and insulates even if it gets wet. Because when it gets wet, the moisture is trapped between the fibers, rather than in them, making it much easier to dry. Its also has other advantages, such as durable, non-allergenic and lower price. That is why most campers prefer synthetic insulation to down insulation for its strong overall performance and friendly price tag.
But the synthetic insulation bag is hard to compress and the synthetic fibers will slowly break down, so you need to take above factors into consideration before buying.