Lorica Segmentata and Auxilia Troops

[Robert Vermaat]

I wonder why the romans had buildt permanent legionary camps which could hold a full legion or even two, if they afterwards distributed a huge amount of legionaries all over the province.

They didn't.
Sure, on campaign legions could be housed in very large, (non-permanent) multi-legion camps. But when the war was over, legionary troops were usually garissoned in a single castra close to the border. The auxilia were spread usually spread out along the border, not all over the province either.