How does early diagnosis help with cancer treatment?

It depends...: It depends on the cancer. Eg... Yes - colon ca, brca (hence screening), lung ca (new ct screen study) likely no - cll rai 0 (early and asymptomatic), indolent lymphomas.

Better prognosis: The idea is if we screen for a cancer and find it earlier then there is a greater chance it's found in earlier stages and cure rate is higher. Treatment of course varies and is different for the stage of cancer. As more and more people are surviving a type of cancer finding it early had a lot to do with those observations and studies.

In theory,: ...Finding a cancer earlier through screening should make therapy more successful but that is not so except in relatively few cancers (breast, colon, cervix, maybe lung) for which treatment at an early stage results in higher cure rates. Also, screening only works for common cancers for which such tests are readily available as well as relatively simple and inexpensive.