Cockpit weather display question

OK, so there I was, training one of our scarce "newbie" controllers at ZTL
yesterday. There was a lot of convective weather and we were busy busy busy
trying to keep people from killing each other in the saddles and holes
between the storms. Anyway, as usual when I am training someone and its
crazy busy, something came up that I'd never seen before. Hopefully one of
you guys can fill me in.

We've got an aircraft at FL170, headed south inbound the Atlanta terminal
area, where the weather is really going bad. I forget the airplane type, it
seems it was a C210 but I can't remember. He's about 150 NM north of ATL.
The pilot calls my trainee and asks for permission to turn off his
transponder for five minutes so that he can "Go download the latest
weather". The trainee had to ask the pilot to say his request again, and
then looked askance at me for an answer. All I could say was "Well, that's
a new one for me, bud. Let him do it and we'll see what happens". His
transponder was only off for a minute or maybe two tops, not the five he
asked for. We were too busy to do what I'd normally do, which is ask the
pilot on the spot for a brief education on what he is up to.

I thought I understood the basics of some of the new in-cockpit weather
tools for GA aircraft. I figured he was getting the latest Nexrad picture
from some online resource. What I don't understand is why he needed to turn
his transponder off to do so. Any help appreciated.

Thanks,

Chip, ZTL