1) Vitamin C is a potent anti-oxidant. When too much oxygen is present in the body, excessive amounts of electrons become present in cells. Cells prefer an even number, and when an extra electron is present, it mutates the cell. The cell, noting the imbalance, discards an electron, which then attaches to a near by cell. The cell that discards the extra electron is now damaged, and the cell it attaches to undergoes the same mutation. The chain continues, until enough cells are affected and you get sick. Vitamin C prevents this chain reaction from occurring by seeking out these excess electrons and lending an electron to them, pairing them up, and thus preventing them from causing damage. This can only happen, though as a preventative measure. Vitamin C cannot break the chain once it starts, so waiting until you are sick is too late. Take vitamin C on a regular basis and it will prevent the chain from starting. It is water soluble, so will not become toxic in the body.
2) Another great fitness reason to take vitamin C regularly is that it is a key player in the synthesis of carnitine. Carnitine is an amino acid responsible for transporting fatty acids into cells mitochondria in order to be used for ATP generation (moves fat into cells to burn as fuel). So by taking more vitamin C regularly, you can burn more body fat. I personally take an EXTRA 5000mg of vitamin C per day and have not been sick since 2012, though I train extremely hard.