Helldritch
Hero
Where are these from? Certainly not from either the MM, Volo or MtoF. Homebrew? A reference to 3.5ed?But why? What's so special about a caster, as opposed to a Black Lotus Assassin (with special kill you abilities) or a Dark Forest Blight Hag (with a withering evil eye)?
Without source I can only guess.
But check the Orc eye of Gruumsh. He has the powers of a 3rd level caster. Just that tells you how many spell slots he has. He also has an other ability. Under 1-3.5ed, this additional ability would not exist and the EoG would have 3HD. With the "blend" he has more HP and has a nice ability. IF you care to go over the other similar monsters, you will notice that they all conform to this way of doing things. This can be helpful for many new DM in modifying their monsters or creating new ones based on what is consistent. In 4ed, monster design was not consistent or should I say consistent in its inconsistency? Especially at high level.