For the priests and seminarians. Holiness cannot be improvised.
It is done for the sanctification of priests, because they are a source of life in the Church, stewards of the Mysteries of God; living instruments of Christ the Eternal Priest that they may be able to carry on in time His marvelous work (cf. PO 12).
Because holiness does much for priests in carrying on a fruitful ministry. And so, for that growth in holiness they will become consistently better instruments in the service of the whole People of God (cf. PO 12)
Holiness cannot be improvised. Seminarians, called by God to be priests, must be helped on their way to sanctification.