What the tool actually does
Screeding gets a slab to the right thickness and roughly level. It does not leave a finishable surface on its own, and it does not need to. A bull float takes over from there, riding across the concrete on a long enough handle that one person can work the full width of a driveway or a patio without walking on the fresh pour. The blade pushes coarse aggregate a little below the top and brings the finer paste up, which is the layer every later step, from edging to brooming, actually works on.
The float also finds the low spots a screed missed and the high spots it left behind. A slab that reads flat to the eye after screeding almost always has small waves in it, and bull floating is the step that takes them out before they set into the concrete permanently.
Timing, and why too early or too late both fail
Bull floating happens once, right after screeding, and the window to do it well is narrower than it looks. Go too early and the float pushes stone straight through a paste layer that has not developed yet, dragging aggregate to the surface instead of burying it. Go too late, after bleed water has started to rise and stand on top of the slab, and floating traps that water under a sealed skin, which is one of the more common reasons a finished surface later dusts or scales.
On a small slab that window is easy to hit because one crew member can cover the whole pour in a few passes. On a larger driveway or a commercial pad, the crew starts floating one end while concrete is still being placed at the other, working the same pace the pour moves at so no section sits too long or gets floated too soon.