The problem is that if you picture what you're doing as firing into the darkness to probe something, you can't be sure if the handful of shots you take actually indicate you've reached "the end" of what you're trying to measure, or just some arbitrary boundary. And that boundary may not actually even exist at the point you measured it, depending on some huge number of factors unrelated to the actual job (for example how much sleep they got last night, or if they haven't eaten this morning and are hungry, etc).