I don't know why they still march. I can tell you that they do not carry ornamental rifles, but rather very large fully automatic weapons. They are members of the military and are fully trained on the use of their weapons.

I once spoke to a man who was a member of the mounted ceremonial guard. They carry only swords. He explained to me the rigorous training that he went through in swordsmanship up to and including how to disarm someone with said sword.

I add this to this comment only to provide information that suggests that the Queen's Guard is not strictly ceremonial despite the costume and ceremony. I was actually quite surprised to see the heavy equipment on the shoulder of the guards when I was there in the late 90's (admittedly as a wide eyed late teen), instead of some kind of ancient rifle.

The 'Queen's Guard' is a rotation of the six infantry regiments - the Foot Guards - of the Household Division, which also includes Cavalry units.

These units were formed as, and still are, regular soldiers with additional ceremonial and security duties. The oldest - the Coldstream Guards*, Grenadier Guards, and Scots Guards** - were formed in the 17th Century, and the Irish and Welsh Guards were formed in the early 20th Century. Their ceremonial duties generally involve parade-ground drill, which is derived from pre-modern battlefield manoeuvres. This meant that through the 17th, 18th, and 19th Centuries the Guards were frequently (seen to be) the most precise and disciplined troops in the army, befitting thier role of guarding the monarch and making them extremely reliable under fire.

While the ceremonial drills have become just that - ceremonial - the Guards are still active-service units and have deployed to every major conflict in which the British Army has been involved for over a century. The Cavalry are divided into ceremonial and operational regiments, but the Infantry are all active troops.

*the Coldstream Guards were not formed as guards to the monarch, as they were raised as a regiment in the New Model Army during the Civil War, and therefore are the only surviving regiment in the British Army to have ever fought against the Crown.

** The Scots Guards are technically the oldest regiment, but they were formed as part of the Scottish Army, and only joined the English Army, from which the British Army traces descent, later. The Coldstream Guards are the regiment with the longest service in the Army, but only began to serve the Crown in 1660 on the Restoration of the Monarchy. This makes the Grenadier Guards, which formed in service to the exiled Charles II in the Interregnum, the senior infantry regiment of the British Army.

Clearly nobody has any knowledge about the UK.