Now, some writers believe that subplots exist only to prolong the word count or basically act as filler content. Well, one thing is known throughout the entire globe: everyone hates fillers.
Sub-plots are stories within stories that can be used to explore and enriches the current world that your characters live in or add further depth into their development as a whole. For example, you could have your main character go back and interact with his old buddies back in his home village... or whatever.
Personally, I use subplots not only to develop characters or flesh out the world; but to also add a somewhat unique aspect to the main story as well. For example, you introduce the hero and the conflict as your main story right? The subplot could explore the antagonist perceptive and offer the "other side of the coin".
That way, and tied to last week's post you create inner conflict and force the reader to make a decision who to support or root for. I also have a rule where I add up all the events that happen in subplots to the main plot as well. Like essentially, there's always gonna be a point where the two collide.
I know this was a short post, but that's all for now, I'll probably do a top tips post on this for next week too.
Until next time,