What you're training
Almost every cadence has a predominant setting up the dominant — but which one? ii6 and IV are the two everyday choices, and they're closer than they look: both put F in the bass with A right above it. The only difference is the note on top — D for ii6, C for IV. This drill trains you to hear that one note instead of guessing from the shape of the phrase.
Two levels, one skill
Level 1 keeps it to the everyday pair: ii6 or IV? Level 2 opens the predominant family up to four choices — the same ii6 and IV, plus ii7 (a fuller seventh chord in root position) and the borrowed iv, a minor subdominant lifted from the parallel minor whose flatted third darkens the whole chord.
Go deeper
What actually precedes V — the census → tallies which predominant composers actually reach for, across real pieces. The theory: predominant seventh chords → covers ii7 and the rest of the predominant family in full. A different way to intensify the approach to a chord → is the other lane here — instead of choosing which predominant leads to V, a borrowed dominant tonicizes a chord that isn't home at all.
Where it fits
Comfortable with chord quality and cadence resolution already? Predominants is the next layer — the exact chord that sets up the dominant pull you already hear, narrowed down from a whole family of options. The full drill ladder has everything else.