In Lure of the Expanse there are acouple of new components one of which is the Excess Void Armour.

it cost 2 SPs, and 2 or 3 space and the only thing it does is Decrease the maneouvrability by 3 and the speed by 2.

there isn't really any reason to buy it at all, I would asume that it was meant also to boost the armour by a certain amount but then again we have Armour plating for that.

Can any help to clarify please?

It increases armor by +3. For related questions, see this thread. It's worth noting that Armored Plating can only be taken once. This upgrade allows you to increase your armor even more for those of who perfer endurance to mobility.

Thank you, good to know.