Mount Doom was one of the reasons Sauron chose Mordor as his home. He knew he could use the fire inside Mount Doom for his forging and magic, which resulted in the forging the One Ring.
Sauron seems to have put his own power into Mount Doom, making it more than just a volcano, similarly to how Sauron's master Melkor had extended his power into the flesh of Arda to corrupt the Valar's creation of the world. As such, when Sauron's power grew, it caused the fires to spring to life.
It is dormant for much of the game until it erupts near the climax and covers Mordor with ash, signifying the return of Sauron.
|Castles and Fortresses|
|Passes and Valleys|
|Outside of Mordor|