Ok since I can never explain this well I'm going to swipe the explanation from another site as to why people buy and need big amps. I always love how this is put to paper on this other site I'm on. I also wonder why it's not known or taught here, maybe to many low power systems here.
Anyway this is form Dan @ AudioAficionado I don't think he'll mind.
One of the main points to consider about amplifier power ratings as it relates to sound pressure levels developed by speakers is dynamic range. No one purchases a MC601 or MC1.2Kw to push a continuous 600 watts or 1200 watts into speakers. What the additional power ratings allow an amplifier to deliver is unclipped and undistorted musical peaks. Nearly all well designed speakers thrive on "clean" power but will fail quickly with clipped output from amplifiers. Reserve output power is essential to accurately reproduce the complex wave forms of music.
Music is not a constant tone and volume is not a constant level unless you are amplifying test tones. Music is a continuous stream of peaks and valleys with an average level that is typically consistent. It is not unusual for momentary peaks in a musical performance to rise above the average level by 10 to 12 decibels. These same peaks are often fleeting but require substantial output power to reproduce without clipping the waveforms. We know that for every 3dB rise of input signal the output power will double. Using this information it is easy to understand why an amplifier with plenty of power becomes necessary. As an example lets say you are enjoying a performance at an average level of 25 watts output and a sudden crash of drums or blast of horns generates a 12dB peak above your average level. 3dB above 25 watts would require 50 watts, 6 dB above 25 watts would require 100 watts, 9dB above 25 watts would require 200 watts, and 12dB above 25 watts would require 400 watts of reserve power in order to faithfully reproduce the peak level of 12dB above your average listening level without clipping the output signal. Knowing this, it doesn't seem like such a stretch to have 400, 500, 600, or even 1200 watts on tap.
Granted this is a casual explanation and doesn't touch on speaker sensitivity, all of which will play into one's choice of amplifier power. None the less, I hope it helps you understand the relationship of input versus output power and why high powered amplifiers may be necessary, particularly as it applies to your speaker's sensitivity and your common listening levels.
Another point to take into consideration is almost all speaker manufacturers measure speaker sensitivity at a distance of 1 meter on axis (39.37 inches) directly in front of the speaker. These measurements are most often taken with the speaker placed inside an anechoic chamber where it is completely silent and there is no reflected sound for the microphone to hear. That's not exactly real world conditions that represent a typical end user. In addition, for each added meter of distance from the initial measurement point the audio level typically drops by 3 decibels. Looking at this another way, picture yourself listening to 96 decibels with one watt of amplifier power while sitting in front of a speaker at a distance of 1 meter. Moving your seat away from the speaker to 4 meters distance without increasing the amplifier output you will now be hearing an 87 decibel sound level. At this 4 meter distance you will need to increase the amplifier output to 8 watts to achieve the same volume level at your ears as was present when you were seated only 1 meter in front of the speaker. Move back 1 additional meter to 5 meters distance from the speaker and the amplifier power requirement will double to 16 watts for a 96 decibel sound pressure level at your ears. Even though a highly efficient speaker that produces 96dB with 1 watt at 1 meter is selected, mating it with an amplifier of 25 watts output can be a detriment or not depending on the distance you place your seat from the speakers and at what sound pressure level you commonly enjoy. Remember this, there needs to be sufficient power reserves (headroom) on tap to reproduce the dynamic differences above average level. If it takes 2 watts to sustain your desired average listening level at your selected seating location, you will still need something on the order of 30+ watts available to handle the dynamic range that is present in many recordings.
Changing the speaker efficiency to 86 decibels at 1 meter with 1 watt the perimeters differ dramatically. If you wish to listen at an average level of 95dB at a distance of 4 meters with this speaker, you will need 64 watts of output power not including reserve power for dynamics. To faithfully reproduce a 12 dB peak above 95dB average level with this speaker you will need 1024 watts, something a MC1.2KW amplifier can deliver in stride, as can a MC601 on momentary peaks. Both of these amplifier's power ratings are considered by many to be exorbitant but in fact they are necessary with this particular arrangement.
Both of these scenarios are examples of how speaker sensitivity, listening distance, and amplifier power ratings correlate into delivering satisfying listening experiences. It is clear that having additional amplifier power is an asset to reproducing undistorted, unclipped music, thereby protecting our speakers from the damage caused by inadequate power. One needs to consider speaker sensitivity, listening distance from the speakers, typical average music levels required and necessary headroom before deciding on amplifier output power.
The original 16 ohm Altec A7-8 Voice of the Theater speaker had a sensitivity of 101dB at 3 feet with 1 watt. This is the kind of sensitivity one needs to look for when planning to use low output amplifiers. I owned a pair of these speakers and drove them to thunderous levels with a pair of McIntosh MC30 mono tube amplifiers. I got major complaints from the neighbors long before I ran out of headroom. It is all about the proper mating of amplifier power to speaker sensitivity and the sound pressure levels you wish to achieve at your listening position. These things must be considered to take full advantage of any particular sound system.