If you live outside the country of Spain, this is a great choice to arrive quickly in cases of special events or a tight schedule.
If you love the fresh smell of the ocean and love boats, this is the mode of transportation for you! Typically takes longer to arrive at your destination but it´s a nice way to enjoy the ocean environment while traveling!
If you live near Spain or the continent of Europe and you don't want to fly in a plane, the subway is the next best thing! Cheap admission and an enjoyable view of the passing lands.