“Access tracking” means that you can actually detect when the object is being used. This can be used for example in deciding which objects to put where in an input stream, to put objects in the order they will be accessed. (Dunno if they do exactly that right now, but that kind of thing is often very helpful!)
Presumably they can also add checks in editor/debug modes where if you were to assign some invalid pointer to the value, you’d get an assert right where it happens, rather than a crash sometime much later when you try to use the bad pointer. (You’d get this from bad casts etc.)
Having worked on reflection systems of various kinds, I’m actually surprised they’ve managed to push raw pointers as far as they have. That would never have been possible without the UnrealHeaderTool, but even with that, it looks to me like an old habit that needs to go away.