The treatment is probably more effective if you capture most/all of the tumor you can. For one, maybe that alone is good enough. For two, theres the fact that many tumors are a heterogenous mixture of populations of cancer cells rather than one clonal mass of cells that are genetically identical. There is a chance you might "miss" and fail to capture a particularly malignant population that might be rare at the time of surgery but might have distant metastasis before long.