The obvious difference between a cadet now, and a Direct Entry applicant is that the cadet has applied at the outset of their career and entered a structured training programme, whereas the DE applicant has waited a few years before applying and done all the training and experience building themselves.
DE applicants can choose their own training provider and pace (for financial or lifestyle reasons for example), and find their own entry-level jobs without knowing for certain whether they're suitable for Qantas for a number of years.
Cadets have to follow an intensive, structured training and experience programme under scrutiny at all times, but have already passed the Qantas selection.
The airline gets a guaranteed steady pool of suitable pilots that can balance any fluctuations in the number or standard of DE applicants.