1) A subplot is not filler.
2) A subplot is used to compliment the main plot, whether it's to add additional obstacles and conflict, add in extra dimensions to your world building, showcase your characters in a more three-dimensional way, or help bring release of tension so the reader can take a breather.
Let's take a look at this closer.
Filler in a book is bad. Your subplot shouldn't be there just to add some extra word count, or because you can't think of anything else to get to your desired book length. So if that is your plan, then back up buddy.
Subplots have so many wonderful purposes. For example, say you have a balls-to-the-walls thriller or action and your character is kicking ass left, right and center. Awesome. But if we don't see another side of him/her then they're gonna be pretty one dimensional. So this is where you can throw a subplot into the mix. Maybe s/he falls in love and it shows his/her softer side (and complicates the main plot as now he has something else to lose).
Okay Fiona, but I don't write action. I'm a romance gal/guy. Same deal. Maybe an ex-boyfriend turns up in the middle of a date, maybe a character loses someone important in their life, loses their job, decides to find a new religion, has a best friend who pressures them to come on a cruise around the world. Whatever it is, use it to showcase their other pressures and personality traits.
Okay, so pretty sure you're with me so far. Are you with me?
Word building - this is not just for fantasy writers. Every book needs a world. They need a physical place in which to exist (duh). So what can your subplot do to help? It can weave in other elements - maybe there's a political uprising that affects your character's ability to navigate in society, maybe a tsunami hits in your romance book washing away (boom boom) the lovers so they need to fight to find each other, maybe you have glorious mountains that your main character explores as a hiding place, maybe, maybe maybe. Go find that big bad world of yours and make it real and vivid so your reader can have a poke around too.
Let readers breathe. High paced actions is awesome, but if that is all they get it will become blah and all the same intensity level. Throw in a change of pace using a subplot and BOOM you have a place to let the reader recover and then a sneaky way to make them gasp when you turn it all up in the head. The romance writer? Well, your characters are all angsty and stuff and loving and stuff, and then booyah - there's a mystery in the village and they're a suspect, or they're promoted and asked to leave the country, or war breaks out. The point is it's not a main plot, it's just something to add a fresh taster of something new.
What I'm getting at here is that it doesn't matter what your genre is, what matters is that you give your reader fully formed characters, diverse situations, additional obstacles and room to breathe in between your major plot points. And this is where subplots are your friend.
Now go find that subplot and hug it tight and call him squishy!