Whats the importance of ruins in game and how to use it for our benefit?

Do you mean the Ruins Exploration locations in the world map or the Ruins that appear when a city has been zeroed?

The first are locations where the Monster Siege enemies spawn from when these events take place. You cannot do anything to these places and they will reset from time to time.

The second are just left over from a destroyed city and will disappear after 15 minutes.