A common misconception is that Shirou Emiya is a Sword. It's an understandable mistake, certainly, but it is still a mistake. The root of this assumption is the fact that both his Element and his Origin are Sword, a circumstance so unique and inhuman that it allowed the existence of Unlimited Blade Works- the ability to project one's own inner reality upon the world at large.
That would be the wrong assumption to make.
You see, when the Corrupted Grail was destroyed, flooding the environment with the pure concept of destruction, the fires spawned embodied that concept. They burned. They destroyed flesh, stone, wood, water, anything they came into contact with, up to and including the base concept of something's existence- They burned the very soul itself.
And it is through these fires that the being which eventually came to be called Shirou Emiya was killed. Not in the metaphorical sense either- as his flesh was burned, his very soul was slowly destroyed in the flames. By the time Kiritsugu Emiya had managed to implant Avalon in his body, he was for all intents and purposes dead, with only the very base essence of his body and soul managing to survive.
And it is with those meger raw materials that Avalon "healed" the extensive damage. Avalon took the incomplete base, and found it wanting. There wasn't enough left to form anything approaching a person. But Avalon is the concept of protection, safety, and health itself. It is the embodiment of the Everdistant Utopia that Arturia dreamed to achieve for her people.
It wouldn't let something so simple as a lack of raw material stop it from achieving Its purpose.
So, Avalon flooded the body and soul both with the fae magic and concept which was inherent in its existence, reinforcing the damaged base, filling in what was missing with it's own "Body" and "Soul."
As you might imagine, this didn't truly heal anything. Rather, it created something new in order to replace what was lost.
Thus, Shirou Emiya was born.
His Origin and Element are Avalon's, his very concept of creation Avalon's, because he, almost entirely, became the living embodiment of Avalon.
And as Avalon is a Scabbard, so too is Shirou Emiya.
A Scabbard's Origin is Sword, as that is what a Scabbard is created for. To protect the Sword, and to ensure the Sword does not harm that which it's master doesn't intend to harm.
As well, a Scabbard's Element is Sword, as the only thing it has perfect and complete control over is the Sword which it contains.
A true Sword would be more likely to have the Origin of Fire or Death, and an Element of Battle or Protection. Only a Scabbard could possibly have an Origin and Element of Sword, as only a Scabbard is made with the explicit purpose of containing the Sword.
And as we all know, the greatest Scabbard to ever exist is Avalon.
Or rather, it was until the Existence (For he could never be called a person, not truly) known as Shirou Emiya was created.
Shirou Emiya. He who was forged with the concept of protecting all from the bite of a blade, of preventing damage to that which is steel.
It is understandable then, that he would be so self sacrificing. A Scabbard by concept of creation is that which is meant to protect others at the cost of willingly taking a blade into itself. A Scabbard by definition is a tool, a tool that lives right next to the razors edge it protects, without fear, without regret, never once retreating from the danger it exposes itself to, for if it does, then that danger may be turned towards those it wishes to protect.
It is understandable then, that Unlimited Blade Works would allow Shirou Emiya full understanding of the blades which he takes into his existence, that it would be filled with all the blades he carries, that he would have all the memories and experiences that those blades themselves carry. For, after all, a Scabbard must be forged in the shape of the Blade it is meant to conceal if it wishes to fulfil its purpose.
It is understandable then, that outside of Shirou's personal reality, all blades would be weakened, for his purpose is to protect. And it is also understandable that the most dangerous place for someone to be, is within the Scabbard's reality, where one cannot escape the blades which he wishes to protect the world against.
There is only one path for a Scabbard, and that is to protect all innocents, never allowing the sword it carries to be unsheathed against those who do not deserve it. That is it's purpose.
Through not understanding this reality, the Scabbard will know nothing but sorrow. A Scabbard knows not victory, and it knows not defeat. To strive for anything else would be to lose it's purpose.
Thus, the Scabbard shall instead toil endlessly to protect the innocent from the edge of a sword, never being truly victorious as it's purpose is not to destroy the blade, but instead to protect it.
In pursuing that path, the Scabbard shall not need any other meaning- for it has always only had one purpose, and will only ever have one purpose.
Truly, the Scabbard is an existence made of Swords.