Purpose of touching tongue to roof of mouth during meditation
As it was explained to me, the purpose is rather mundane. During some kinds of meditation, esp. when one penetrates lower chakras, there is lots of saliva generated, swallowing which can be quite a distraction and interfere with abdominal breathing. Raising the tongue makes most of saliva flow back into the throat instead of filling the mouth.

P.S. in a commentary to ancient Chan text "Lung Chung" (The Brief Precepts) it says that pressing the tongue to the roof of the mouth eliminates the objects of discursive (speech-type) thought.