The clip is secured in one place so that it cannot slide off the core (e.g. when exposed to mechanical vibrations).2)
Additionally, because the clip sits in the recess, the whole setup occupies slightly less space when assembled, than for a comparable core without a recess.
The cores and plates with recess are usually marked with additional “R”, or “S” (slot) for plates. For instance, the core could be E14/3.5/5/R and the corresponding plate PLT14/5/1.5/R or PLT14/5/1.5/S.3)