Own a building at the port
A settlement or city on one of the port’s connected endpoints gives you access. A road merely reaching the harbour does not. A general port offers three of one resource for one different available resource; a specialist port offers two of its matching resource for one.
Specialist access is narrow. A wood port improves wood exchanges but does not make two brick exchangeable at the same rate. The bank must also hold the resource you want to receive.
Look for a repeatable surplus
Suppose a strong wood position regularly leaves you with four wood after your road needs are covered. Without a port, four wood buys one missing card. A wood port can convert the same four wood into two missing cards through two valid bundles.
That is useful flexibility, but not free production. If every wood is already needed for roads and settlements, exporting it may slow the plan the port was meant to support. Count your own demand before labelling cards spare.
Include the cost of reaching the coast
If the port needs two roads and a settlement, its access costs three wood, three brick, one wool and one grain. The new settlement also earns a point and may produce resources, so evaluate the whole position rather than the discount alone.
A poor coastal corner with weak production can still be useful late in the game when it unlocks a needed exchange. As an opening placement, the same corner may sacrifice too much production before the port has anything to convert.
Choose flexibility or a strong speciality
A general port suits a hand that accumulates different kinds of three-card surplus over time. A specialist port is stronger for its one resource, especially when a good city supports that resource. Neither replaces the need for legal building sites.
Compare the port exchange with player offers each turn. A willing partner may offer a better rate, while the port gives you an alternative when nobody wants your surplus. That fallback can improve your negotiating position as well as your purchases.