Dolphins are usually installed to provide a fixed structure when it would be impractical to extend the shore to provide a dry-access facility, for example, when the number of ships is greater than can be accommodated by the length of the berth/pier.
To provide a dry-access facility when it's not feasible to extend the shoreline, dolphins are often employed. This is particularly useful when the number of ships exceeds the berthing capacity of the pier or berth.