A single lighthouse tells you where something is. It marks a hazard, or a headland, or a harbour mouth. What it cannot tell you is where you are, because a light seen from three miles looks much like a light seen from two, and a bearing taken from a moving deck in poor visibility is a guess with decimal places.
Two lights solve it, provided they are in the right relationship. Put one low and near, put the second higher and further back, and align both on the centreline of a safe channel. A mariner steers until the two appear stacked, one directly above the other. At that moment they are on the line joining the towers, and the line has been surveyed to sit in deep water.
Off to one side, the lights separate. Which side they separate towards tells you which way you have drifted, and by how much. There is no instrument involved, no calculation, and nothing to fail. The information is delivered by geometry straight into the eye.
That is why the arrangement survived every technological shift that killed off its contemporaries. It is not a signal that has to be interpreted. It is a picture that is either correct or visibly not, and a helmsman can act on it in the time it takes to look up.
The formal name is a leading line, and the two towers are the leading lights, or in American usage the range. The near one is the front range, the far one the rear range. Everything interesting about how they perform comes down to the distance between them and the difference in their heights.