If she only had one set of arms with those giant guns permanently attached to them, she would be incapable of actually doing anything with her hands. She'd need help to pick something up off the ground that is right in front of her.
With the guns on a second set of arms that mirror the motion of her natural arms, she can simply un-synch the arms if she needs to do something with her hands.
Let's be sensible about this - what could she possible be doing in the middle of combat that requires the use of hands and the detriment of possibly unsyncing the combat arms that wouldn't be better served by waiting until combat is done and simply unmounting the weapons from the arms, or using the hands that are built into the arms?
The hands doing the manipulation wouldn't be her real hands anyway, they are Crisis Suit-style hands.
If you have no armament on the real arms, then there's no reason to have them out there.
It's a dumb design - it makes less sense than the Dreadknight, because at least for the Dreadknight, there's no indication that the Imperium have the AI
/tech-capability to use totally automated arms with a pilot inside a pilot compartment, unlike the Tau, which is exactly how Crisis Suits work.
Also, going by the positioning of the arms in the quad configuration, there are going to be positions that the real arms might be in that make it unnecessarily complicated, if not impossible, for the AI
to account for working around that could potentially delay use of the weapons in combat - the real arms being encased in armor and forward of the artificial ones will be in the way and doing nothing as currently armed. It's a detriment, not a benefit.
You have real unarmed arms making the combat process more complicated, and even a few additional fractions of a second of time for the AI
to calculate how to move around them is time wasted and the potential for an enemy to start getting a shot off, and possibly move without being hit in return (especially if the real arms move to protect the body, possibly forcing the AI
to once again recalculate movement to keep a shot on target).
At the very least, the real arms should mount secondary weapons, or it makes no sense to have them out.
Truthfully, it doesn't make a lot of sense for any of her real appendages or head to be exposed outside the central pod, but I realize that they were going for something other than "Yet another Crisis Suit Commander" - just doesn't make a ton of sense.
And if they aren't going to arm the real arms, then they should have make them smaller, say terminating where her real arms end so that they are easy to get out of the way of the artificial ones and can be defended more readily.