Why do I need a blood transfusion?

I've recently been rediagnosed with osteosarcoma in my lungs after a biopsy. That was done by keyhole surgery so I don't think I would have lost much blood. But now Ive been told to go in to get a boost of blood in the next few days. Is this just a precaution before I start chemo? Or can just having cancer in you make you need more blood?