A harbor gets me another trade route which is certainly useful. Late game cities can add trade routes for science, and a place to upgrade units for your military if going domination, or just preventing an AI culture/science victory. They can also add amenities to keep unrest down as your expanding. Captured cities have even more production issues with occupation penalties and generally crappy existing districts with few land improvements. There is a cap on number of trade routes, you only get one per city so you need more cities for more trade routes. Also only 'good' cities can host a trade route. A new city returns almost nothing on trade unless you build districts.