Why were bayonets useful during World War 1?

They weren't. Every foot soldier in every army had one. Few were able to get close enough to the enemy to stab anybody with it though. As long as the soldier had ammunition for his bolt action rifle it was far easier to shoot the enemy, as fast as possible before they shot him, than to try to close in and poke them with the bayonet. They were more useful as a big knife around the camp, or for spitting and roasting meat.