Used to introduce an additional thing that reinforces the point you are making, typically something obvious or notable.

उल्लेख करने की भी आवश्यकता नहीं

English Usage: The garden was full of beautiful flowers, to say nothing of the breathtaking waterfall.

Hindi Usage: बगीचा सुंदर फूलों से भरा हुआ था, तो वहां के मनमोहक झरने का तो कहना ही क्या।