Synthetic fabrics are a man made material and have some great benefits.
They are ideal for beginner to itermediate walkers and hikers. These boots and shoes tend to be cheaper than their leather counterparts.
There are 3 main advatages to synthetic materials, the first of which is breathability. Haveing a breathable boot keeps your feet cool and comfortable which in turn makes them idea for warmer climates and long periods of walking.
Some of the higher end boots use Goretex which is hard wearing, breathable and water proof also.
The second benefit is weight, sythetic materials are light which reduces weight on your feet whilst walking cutting down fatigue.
The third and final advantage is that synthetic boots don't need to be worn in. Meaning they are good to go out of the box.
The downside to synthetic fabrics is that they are generally not as hard wearing as leather boots. Although Goretex in particular will easily stand up to most uses.