If your tent has a good fly, there is no need to put a tarp over it.

I don't do it myself.  But I also wonder if there is a science behind it. Could it be to protect the fly from uv damage? That's the only reason I can think of

I use a tarp over my tent when backpacking and canoeing even though my tent fly is very capable of withstanding even heavy rain. Here are several reasons why:
- Ever pitch a tent in a downpour? The tent gets wet. Putting a tarp up first gives you the chance to get the tent up while both you and the tent are shielded. Of course, when you are striking camp and it's raining, having the tarp overhead allows you to pack the tent and fly up nice and dry. The wet tarp goes in a separate bag and is no problem to dry out later.
- If the tarp is large enough (mine is a 9 x 12), you can get out of the tent in the rain to do something close to the tent and not get wet.
- if the rain is coming down horizontally, a large tarp over the tent pitched properly will keep some of that rain from getting in under the fly.
- When faced with a strong cold wind from a single direction, a large tarp that acts as a wall can actually keep you warmer.
- In hot weather, having a tarp a few feet overhead can keep the tent from getting very hot inside, especially in locations exposed to direct sunlight.

Just some food for thought.

Only used a tarp once. Found it a pain. I prefer a tent with a large vestibule to sit dry. Problem is some of the tent flys aren't very waterproof, for that reason I have graduated to 4 season tents with a floor coating of 10.000 mm waterproof ness and a fly of 5000 if I can find it. It does not add much to the weight of a tent ( but it does effect price) and saves me having to worry about where to put it in case of a campsite with the unfortunate placing of trees.

Thanks so much for all the responses.  It looks to me like it is less a matter of a technique I wasn't aware of as it is the use of a clever idea.  It appears there are both "for" and "opposed" camps on this topic, which I did not anticipate.  Our tent has a small vestibule, (I guess I would let the boy go in there first in a driving rain, and kind of encourage him to get a move on), but for our purposes I'm thinking we'll stick with using the one tarp we have over the kitchen area if we've got unpleasantness.  Maybe we'll experiment a little bit depending on the circumstances.  I don't want to start carrying two tarps though.  For one thing, they are not cheap and with all the gearing up I've done since January my wallet has taken enough of a beating..  Also, while they aren't super-heavy, they do add weight, and they do take up space, both of which are at a premium, especially when daddy is doing the bulk of the carrying. 

Again - thanks for all the responses!