my understanding is that a big part of determining the stage of a cancer is based on tumor size and spread.

https://www.cancer.gov/about-cancer/diagnosis-staging/stagin...

I think most treatment is focused on killing/removing tumor cells and reducing tumor, so the earlier stage a cancer is detected, the less tumor have to remove, and the better the chances of removing all the tumor so there's nothing left to grow, spread, or metastasize beyond the initial tumor site.