I understand that joining cities with workable tiles creates borders that give lots of defensive benefits, but are there any other reasons for doing so?

I want to build cities along the coast, which will be joined by harbours, but I am unsure if not linking their tiles will cause me problems.

Cities naturally 'grow' tiles with culture points. The map tiles around your city become the colour of your civilisation, which means that your city can 'work' those tiles. A city also has the ability to purchase these tiles at a cost of gold.

You can join cities by linking their workable tiles together. I understand that doing so gives a territorial border, but are there any other reasons for doing so?

I want to build cities along the coast, which will be joined by harbours, but I am unsure if not linking their tiles will cause me problems.

The image below illustrates three cities that are joined by workable city tiles: