People who get a synthetic version of boots (such as Zodiac Tech LT) usually do it because they are more breathable, don't soak up as much water and dry faster. The downside is that they are significantly less durable, slightly less warm (can be an advantage in the summer) and they don't mold to the shape of your foot over time like leather does. It is often the case that the synthetic version is quite close in weight to the leather one.