In March, I finished six months of chemo. It was 'preventative'. However, in truth, there is no such thing as preventative chemo. Sure, some are easier, some don't make you lose your hair, but all are intrusive and lengthy. Chemo is chemo is chemo.
For most, surgery will remove the cancer. However, it is possible that during the time the tumour has been growing, it could have metastasised. This means it could have shed a cell, possibly more. (Remember, a single cancel cell are minuscule, they cannot be seen or show up on a scan). What preventative chemo is given in the hope of stopping/killing any stray cancer cells that shed (so haven't been removed during surgery) and have spread through the body, are laying dormant, and could reawaken at some point in the future. Preventative chemo is given to stop the cancer metastasising elsewhere.