Every other mode draws its text from a fixed list, which means someone who struggles with P spends most of a session typing letters they already know. A drill turns that round: it asks your key profile which letters are actually costing you time and builds the text around them, so the keys you are weakest on appear several times more often than ordinary writing would produce.
The words are real words rather than nonsense strings. Drills of the "ppp lpl plp" kind concentrate a key further, but they lose the finger transitions and the rhythm that make typing feel like typing, and the practice transfers less well to actual writing.
It follows you: once a key stops costing you time, the drill stops asking for it and moves on to whatever is now worst. On a first visit, before anything is known about your hands, it serves a general warm-up instead.
Also try: your keyboard heatmap · open practice