It says that T&D rockers have internal oiling. What makes that different or better?

Yes, most T&D rocker bodies (except SportComp) have oil passages that enable engine oil to constantly pressurize the trunion bearings and the rocker tips. This feature gives a longevity factor to T&D rockers that the competition does not provide. It also cools the bearings and shafts. There is also an orifice in the adjuster screw that maintains oil to the interface between adjuster cup and pushrod end. This part of the oil passage must remain open and precautions must be taken to maintain adjuster position (another reason that pushrod length is critical).
The rocker arm should not be operated with the adjuster screw more than one turn up or down, from the initial adjuster position. Doing so will cut off the flow of oil to the rocker arm.