Well, due to the new renderer in A7, this will become a very relative statement since the rendere has improved speed and a optimized culling algorithm. Though, there are situations, where entity managers would be handy: imagine large outdoor areas and random weed & bush models. Instead of having up to hundred entities being active at the same time, only the half will do the same (with LOD, of course).

Well, you just need to know where those managed entities are positioned and some additional properties, like scale and rotation, for example). Then, during runtime you could analyse the position and angle of the camera to place the entities (simplest method: radial check) or prediction (analyse camera's pov and relative motion and rotation speed).

Though, for beginners, this isn't trivial. Nevertheless, there is a tree script to be found on the Acknex Resource site which does this, I think. Have a look into that code.