I suppose the tradition dates back to times where it was a miracle for an every-day family to have food at their table. It's also a practice not exclusive to Christianity as some cultures do so as well. In Japan people commonly say "Itadakimasu" before eating a meal. But I don't see the point of thanking God for food, even if you believe in him. You have food because you or someone else worked for it, and we have plenty of it in this world after thousands of years of farmers learning by trial and error. If any deity had an effect of bringing food to a table, we'd have no starving children or hungry adults.