Where a subclass is a branch of a class, a prestige class is a convergent class: a specialization multiple classes can share, or a path that different classes travel toward and meet at.
Two necromancers, one from a Warlock background and one from an Arcanist background, can lead to interesting narrative moments. There could be a whole tribe of necromancers, shamans or grove keepers, all dedicated to a similar cause and study but coming from different walks of life.
Prestige classes have their own Prestige Class Feats (see Feats).
Prestige classes
- Dread Necromancer
Rewarding pure classes
The designer wants some kind of reward for players who stay a strict base class without dipping much (or at all) into subclasses or prestige classes.