Was the point of light houses to attract trade?
> The Lighthouse, or Pharos of Alexandria... stood more than 100 meters tall above the island of Pharos, guiding ships safely through Alexandria’s treacherous coastal waters
Note that making navigation safer would also benefit trade. Given the choice of landing in a port that was difficult to find and one that had such a lighthouse as a guide...
If you know your location you can do a much better job of avoiding known hazards.
I just realized that if there's 3 of them, it'd constitute an ancient GPS isn't it. Or even only two?
You might be interested in "lead marks" which are two signs visible from a distance. They "lead" ships by providing a straight path through difficult passages when the two signs line up. Very simple and effective.
Note that modern lighthouse lights either rotate or blink, the flash frequency is unique and marked on charts to further help identify location at night.
Monument lighthouses have an extra purpose: they project power and wealth. Merchants know this place is Important. Like modern day monuments, whether people need a giant expensive building/statue/obelisk to learn this or it’s just a vanity project for the ruler is a matter of opinion. People aren’t really all that different over the last 2000 years.