A big growth area for dark fibre at the moment is Data Centre Interconnect (DCI). This is where there is a direct point to point connection between two data centres. By taking control of the equipment between the data centres you can manage and control the link as if it's just an extension of your data centre network. Plenty of capacity, easy integration, control, encryption if you want it, are all benefits of the dark fibre approach.
Fibre to the Antenna (FTTA) is another growth area, as we progress to higher capacity 4G and 5G mobile networks, then each base station needs the capacity of a fibre connection to meet demand.
Most National Research and Educational Networks (NRENs) that link together Universities and research establishments use dark fibre. These include networks like JANET in the UK and DANTE around Europe.