The flamethrower sometimes was used to shoot out liquid gases or flames. It was used to help kill people without you having to leave your trench. Even though you are able to use the flamethrower it doesn't mean that is the only thing you should use during the war. You have other tools to use so you should use them.
A weapon that was a serious threat in the Great War was the submarine. The United Kingdom relied heavily on imports to feed its population and supply its war industry, and the German navy hoped to starve and weaken Britian armies by using U-boats, a type of submarine used in the war to attck merchant ships.
Machine guns are usually designed to fire rifle cartidges typically at one hundred rounds per minute. They are used to fire at enemies more rapidly than you could manually. Each individual soldier was given a gun and they used it more than any other items they had because when they went on the front and had to perform a quick reaction to one of the enemies you would not use the flamethrower you would use the machine gun.