A good pair of floorstanders will give about an extra octave of low frequency response, down to around 30-35 Hz. Modern film soundtracks can go as low as 10-13 Hz. In that range from 10-35 Hz is where you get the big impact effects of explosions, car crashes etc.

If you don't have a dedicated sub, those low frequencies are directed to the speakers, which then have to work harder to handle the full range of signal. If a sub handles the lower frequencies, the load is reduced on the remaining speakers which can focus on producing the mid and high range instead, reducing distortion and enabling cleaner, louder playback levels.