>>9627658>but those are just there in case Earth is attacked by... I dunno, the Namekian gods or something
I think you are referring to Counter Guardians. Which are different from Servants in a sense. They're both Heroic Spirits, but one is different from the other.
Apart from those who became heroes by their own strength, there are those who can make a contract with the World (here, Alaya, AKA Arayashiki) to "become a hero in this world in exchange of becoming a Heroic Spirit after death".
Those who became heroes through a contract with the World, and those who have no legend and were not observed or believed in, join the big classification of "Guardians" and are used by a mindless "Counter Force" (hence they are usually referred to as "Counter Guardians"). Heroic Spirits who became heroes through their own power are not used by the Counter Force.
In Fate, Emiya is one of those Guardians.
Guardians have no free will, they are only a "force". They come out only to protect humans, when a "factor of destruction of the world" appears; they are the extermination weapons that will annihilate that factor. If humans happen to be the "factor" of their own destruction, then they become the target of that extermination. In brief, by killing all humans on the place they appear at, they save humanity as a whole. They are anti-heroes, cleaners called out when something is causing harm.
The other Heroic Spirits are existences with a higher divinity ranking, closer to the existence of a planet than of a human, therefore they do not have to become Guardians.