When the original firearms were introduced it soon became evident that there was a gap between the firing of a shot and the next chance to fire that would potentially allow the enemy to advance. One way to overcome the problem was the 3 ranks in the "British Square" where each rank fired, reloaded and made ready in turn.
Each soldier could fire 3 rounds per minute so 3 ranks as described could fire 9 rounds per minute. However the range of the bullet was less than 100 metres so the enemy could get quite close without fear.
Although partly successful it was not a total answer.
Hence the introduction of the bayonet. It was/is basically a knife (or sword) attached to the end of a rifle. Originally they were "plug" bayonets as above. The hilt was smooth and designed to fit into the barrel of the musket. However that meant that the rifle could not be reloaded so the idea was short-lived. The socket bayonet was then invented. It had a ring that fitted around the muzzle of the rifle so that the bayonet could remain on the weapon and the rifle could still be fired. When muzzle loading was replaced with the cartridge rifle this was even more successful.
Over the years, the terrain and tactics have had an impact on the need or use of the bayonet. In jungle fighting and in house to house fighting a long bayonet on the end of your rifle can be a hindrance not a help.
Rifles have become shorter, have been replaced by carbines or assault rifles and in many cases now have folding or removable stocks.
The bayonet has basically been replaced by the fighting knife, at least in the western world.