Subplots - Why Do You Need Them?

This week I'm learning about subplots and why I need them. 

Main Plot -- Events that make up your main story.

Subplot -- Supporting storyline that deepens the overall story, adding complex layers or understanding to the main story.

So, based on the definition of subplot, I want subplots because they will add "spice" to my larger main story. It would be like the hills and loops of a roller coaster. The ride might be good and fast without them, but with them, the rider gets an even better trip!

If you'd like to learn more about subplots, check out these great posts. Subplots Screenwriting Tips (don't be scared off by the "screenwriting" bit, this has great tips for any fiction!) and The One Subplot You Really Need.

Here are my plots:

Main Plot: Library friends, thrust into troubled kingdom, must protect maps that hide powerful treasure from an insatiable tyrant and his demons, and they learn to treasure what matters most: each other.

Subplot 1: Rogue Half-Demon also wants to find the treasure, so he can use its power to cure himself.

Subplot 2: Peasant boy finds out he is the one in prophecy that will re-unite/save the troubled kingdom.

Subplot 3: Tyrant's henchman leads the demons on hunt for the maps.

So, what do you think about subplots? What problems or successes have you encountered with using them in your writing?