Reduce edema! Brain metastasis induce leaking vessels, swelling or edema that increases mass effect and symptoms. Dexamethasone is a catabolic steroid, not anabolic, reduces swelling. It also turns off the stimulus to make our own cycling corticosteroids, so it must be tapered and never discontinued abruptly. These provide comfort, reduce symptoms.
Brain edema. Intended to reduce intra cranial pressure, relief symptoms ( head ache etc )steroids will not shrink the tumor, good palliation in the terminal stages.