All 3 bikes i recently bought, 68,70 and 71 DT250's have this foam piece under the gas tanks. Many bikes i have worked on in the past didn't have it, most likely long gone over the years. 2 of these the foam was glued or stuck to the frame. Was hard to remove from frame.
I always assumed it gave more support for the gas tank because you can definitely feel it when sliding the tank back on. 

The '68 DT1 I am working on had remnants of foam glued to the top frame rail. I assumed it was there to dampen things and support the tank better. I will probably try to replace with closed cell foam to avoid water saturation. (who am I kidding, the only water it will see will be from the garden hose... )

Isolates cables and wire harness from chaffing tank.

cliff

My vote is more support for tank.