What makes your audio visual experice
Sub-woofer don't have to be huge or even be seen. They can be as shallow as 5" and be hidden in may places, yet still sound awesome.
Speakers are generally not placed in vehicles in an enclosure or an appropriate way to produce clean, loud, and the full range of low frequencies. So speakers generally don't do a good job of reproducing bass. That' where sub-woofer's come in, they are a key point of any quality sound system to complete any audio experience.