The gap it fills between other tools
A bull float levels a whole slab in wide passes, but its long blade cannot follow a curve, tuck into a corner against a wall, or work a strip only a foot or two wide without leaving marks where the blade catches an edge. A fresno covers that gap. The head is roughly the size of two hand trowels put together, mounted on a handle long enough to work from a standing position, so one person can finish a narrow section without kneeling in wet concrete or stepping onto the slab itself.
It shows up most on jobs with a lot of perimeter relative to open field. A patio that steps around a hot tub pad, a sidewalk against a foundation wall, the strip along a garage floor where it meets the slab edge. Those are places a bull float either cannot reach or would do a worse job than a smaller, more controlled tool.
Where it fits in the finishing sequence
Fresno work comes after bull floating and generally around the same stage as edging, once the concrete has firmed up enough to hold a smoother finish without the tool sinking in. On a plain slab it is often the last hand tool before the surface texture goes on, whether that is a broom finish, a wash for exposed aggregate, or stamping.
It is a hand tool with a long handle, not a power tool, which means the pace of the work is set by the crew member holding it rather than a motor. That matters on smaller or oddly shaped areas where a power trowel would be clumsy anyway, and it is one more reason a finish crew for a stamped or heavily detailed pour runs bigger than one for a plain rectangle.
The shapes that call for it most
Curved patios, walkways that step around a fire pit or a hot tub pad, and any slab with a lot of perimeter relative to open field lean on a fresno more than a plain rectangle does. A straight driveway with square corners can be finished almost entirely with a bull float and an edger. A patio built around a grill area or a stoop that ties into a set of steps needs a crew member working a fresno along every curve and corner by hand, which is part of why an irregular shape takes longer to finish than a simple one of the same square footage.