To help ships navigate. Being 100 meters tall it would be visible from quite a distance off shore, something like 36km or 22 miles, before land itself could be seen.

> 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...