Why the Queen's Own exists
The Queen's Own exists in order to provide physical security to the sovereign and to members of the royal family, to provide "area" security for Mount Royal Palace and for other royal residences, and to provide the in-house security functions required for both of those major areas of first possibility.
The Royal Manticoran Army exists not simply for garrison duties, but to provide the mechanized "muscle" for sustained, heavy combat. Moreover, the Army's "Atmospheric Command" could actually be thought of as the Royal Manticoran Air Force, as it is responsible for both atmospheric and near-space security for the planets of the Star Kingdom. In addition, it is the Army which provides the personnel who man the heavy ground defense weapons against both atmospheric and extra-atmospheric attack.
The Queen's Own actually consists of four battalions: one battalion for each of the Star Kingdom's originally colonized planets, plus the School Battalion. The School Battalion's functions are to provide for the Queen's Own's own training requirements, and also to provide elite "op forces" for the Army's (and Marines') training commands. Each battalion of the Queen's Own has an additional company, bringing its strength up to 735 men, or a total of 2,940 men. Membership in the Queen's Own is available only to men and women who have thoroughly proved themselves in one of the combat arms before recruitment.
Like the Army itself, The Queen's Own contains its own atmospheric component. Most of its personnel, even on security detail for the monarch herself, are normally configured as light infantry, but each battalion has its own integral heavy weapons company, and powered armor is available to it.
Although successful assassinations of monarchs -- and of other members of the royal family -- have been rare, attempted assassinations are not unheard of. Most of those attempted assassinations are detected and defeated or deterred well short of execution by the Queen's Own and other civilian and paramilitary organizations like Palace Security and a civilian police SWAT teams. The primary reason for sting ship escorts and heavy weapons are that the technology available in the Honorverse provides all sorts of avenues whereby attacks -- especially suicide attacks -- can be carried out. For example, one of the attempted assassinations which was thwarted consisted of a suicidal attempt to ram the monarch's armored air limousine with a cargo shuttle. It failed because the sting ships were available to intercept and shoot it down. Handheld anti-air weapons do not have the same kill probability as specialized fighter aircraft, and even if they did, would be useless for intercepting such an attack while the monarch was in flight, since no one on the ground would be able to keep up with them or to provide a moving bubble of protection.
In addition to all of the above, the Regiment is the Army's premier ceremonial unit and has been a part of the Star Kingdom's military traditions since before the founding of the Star Kingdom itself. The original parent unit which became the King's Own for Roger the First was formed to provide security for the colony's chief administrator and critical government offices.
"Oh, bother!" Pooh said as Piglet came back from the dead.