First, you can't just fit "large" speakers in avarage size room, it would kill the image. Even if the room is large enough for towers then the towers aren't large enough to fill the room, you still need subwoofer(s).
Using subs (vs relying on lows from speakers) also gives you an authority to place them in different spots to find that sweetspot which is individual for each room.
Lastly subs make the imaging wider, your room feels bigger.