system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] Why does the Adeptus Mechanicus need a fighting force?

I have heard murmurs of previously unseen units at their disposal and was wondering why they need a unique army. surely an army of guardsmen could defend the Techpriests...

They are so close to Holy Terra surely if there was any trouble we could send help pretty quickly.
gpt
Because they had a fighting force before coming into the Imperium's fold. The Mechanicus' military is, essentially, a relic of the times before the Emperor came. 

They are also, for lack of a better term, a personalized army that the Mechanicus can do with as they please. They can't give direct orders to Astartes or Imperial Guard units, but they can have their Skitarii do whatever they damn-well please. When you are under the protection of two of the most powerful forces in the galaxy, having your own mini army to fuck around with must be kind of awesome. 

Are these units used extensively? Not at all. In fact, many military individuals will never come into contact with a Skitarii.

So to answer your questions, they DON'T need a unique army. But they already have one, so why not keep using it?