The purpose of the brining is, in part, to remove any muddy flavours. Also, when you add the spices and herbs to the brine, the point is that the brine will get drawn into the fish and take with it some flavourings. The salt and sugar, which is usually added post-brine for a bit, pulls more water out of the fish and pulls the salt and sugar into the fish. This helps to firm up the flesh a bit (so that it flakes nicely after it's smoked) and to pull the salt and sugar into the flesh. Often, the excess is washed off the fish, so only that which is drawn into the flesh remains to provide the flavour during cooking.

- Jeff