Staff is the mechanism that turns a small stable into an operation. You hire employees for ongoing care and training, and every hire lets you cover more ground than you can physically manage alone.
The confirmed roles cluster around two headings that match the game's two long-running labour sinks: ongoing care — grooming, feeding, hoof picking, treating scratches and injuries — and training — running exercises, building strength and confidence, and working through desensitization. In a game where every horse needs daily attention, the practical effect of a hire is that you can add horses without adding your own hours, which is exactly what you need to grow past the single-trusted-mare starting point.
Care outranks training in the early game, and the reason is arithmetic. Care is per-horse per-day; training is per-horse per-progress-bar. The horses that need care are all of them, immediately, and every one of them contributes to whether you can compete at all. Staffing care first means the horses you already own are working at their potential; staffing training first means you are investing in a horse that is being neglected while you train it.
Toward the end of the game, staff is no longer the obvious buy. Every hire competes with building expansions for the same money, and a well-built stable can cover a lot of routine work with fewer people. The choice is between a bigger operation with fewer hands per horse and a smaller one with more attention per horse — and given how much of the game's depth is in the care layer, the second option is often better than it looks on the spreadsheet.