What good is taking mega doses of vitamin C for?

Nothing. Since vitamin c is a water soluble vitamin, your kidneys excrete what you do not use each day. So if you are buying vitamin c, you are just producing expensive urine.
Dangerous. Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.