The first pass, not the last
Concrete comes out of a chute or off the end of a hose in a rough, uneven pile, higher in some spots than others. Screeding is what turns that pile into a flat plane at the right thickness, sitting level with the top edge of the forms. Two people can screed a small slab with a straight board pulled across the top of the forms in a sawing motion, working the excess ahead of the board and off the end. A bigger pour or a stiffer mix often calls for a powered screed with a vibrating head, which helps the concrete settle into the corners as it strikes off.
Screeding and floating are two different jobs
It is easy to hear screed, float and finish used back to back and assume they are one motion. They are not. Screeding sets the grade and removes excess concrete, nothing more. Floating comes after, once the water sheen has left the surface, and its job is to consolidate what is already at the right level, working a bull float or a mag float over the same area to knock down ridges and press small voids closed. Skipping straight to floating without a proper screed pass leaves a slab that is smooth on top and wrong underneath, thick in some spots and thin in others.
What a bad screed pass costs you later
A slab that comes off the screed out of level is hard to fix afterward. Water pools where it should be running toward a drain or off a patio edge, and a floor meant to hold a car lift or a workbench ends up with a low corner nobody notices until something on it does not sit flat. Getting the screed right the first time, and checking fall in more than one direction while the forms are still adjustable, is cheaper than any fix that comes after the concrete has set.