My floor was 21 feet wide with 7/8 inch variation from the highest point to the lowest. The manufacturer spec was 3/16 over 8 feet. Before I could fix anything, I needed to find where the high spots and low spots actually were. That is what the laser level is for.
One clarification before starting: a laser level checks flatness and maps elevation across a floor. Self-leveling compound flattens it. The laser tells you where the problem is. The compound is how you fix it.
Flat is not the same as level
A floor can be level and still fail the flatness spec for most flooring products. Level means the surface is parallel to horizontal. Flat means no humps, dips, or ridges over a given span.
How to map the subfloor with a laser level
Mapping gives you a picture of where the floor is high, where it is low, and how large the transitions are. You need this before applying any compound or shimming any joists.
Set the reference height
Mount the laser on a tripod near the center of the room. Let it self-level. The horizontal beam is your reference plane.