Why would my mother's doctors put her on steroids for her lung cancer?

Two Main Reasons: The two main reasons lung cancer patients end up on steroids include 1) using the steriod as an appetite stimulant and 2) treating inflammation of the lungs caused from radiation treatments (called radiation pneumonitis).

Dr Kuitems agreed

Dexamethasone: The other steroid used commonly in assocation with brain metastasis, and the ensuing swelling. These steroids are "catabolic" - break down muscles, cause muscle weakness, unlike the anabolic tetosterone derivative steroids used to build up muscles, and no established role in oncologic practice.

Other reasons...: I often see people before surgery whose pulmonary function tests (pfts) are poor. To try to maximize lung function prior to surgery, i will give a trial of steroids and repeat pfts after ~2 weeks. In some instances this will improve pfts to the point where a surgeon feels better about offering resection. People with lung cancer require a certain amount of lung function reserve to tolerate surgery.