For 75-85 dB a few watts are enough so ...

If most of the time we are using just a few watts from a given amp to get a SPL of 75-85 dB from the speakers why some people said things like "with this speakers you need al least 100-150 watts to make them sing"??
A more powerful amp tends to offer advantages at both low and higher volumes. You may have noticed with some amp/speaker combinations when you turn up the volume the overall sound improves -more open, detailed etc. With a more powerful amp, even at low volumes things open up and are more detailed - equal to playing at higher volumes.

The other issue is some speakers (like mine) are power pigs - 84db, 3/4 ohm load. While you can get sound out of them with a low powered amp (I even got them working with a 13wpc fatman tube amp), the sound is dull with no dynamics. More power fixes that at all volumes.
-some people like to listen louder
-some of us have hearing loss...may need to turn it up to
hear the highs better as you get older
-some have large listening rooms
-some use it as an excuse to need to buy a bigger amp
-some music sounds best cranked (ie. Van Halen, AC/DC, etc.)
-75db isn't really loud enough for our ears to perceive the music as "flat" (see Fletcher-Munson curves)@
-sometimes playing louder than usual is a way to compensate for an uninvolving speaker
I agree with Meiwan and Ddd1. Speakers that are not efficient take an amp with a lot of current. A 84db speaker will require 2-4 watts (depending on room size) to play at 84db from the listoning position.
Most amps with 10-20 watts do not have the current to drive the hard load that the 84db 4ohm speaker requires to sound full and dynamic. It will reach the SP's but lack in punch and dynamics.
Hope this helps.

I think there's a lot of fuzzy thinking and bad math when it comes to this subject. 500 extra watts doesn't make the first watt any better or worse. 1 watt= 1 watt.
My experience differs from Meiwan`s, a high quality low powered amp will more often then not sound much better at low and normal volume levels. Better in terms of resolution, inner detail,microdynamics,nuance and natural tone. I do agree that for inefficient, high current demanding speakers, high power amp are mandatory. A high efficiency/easy load designed speaker sounds superior with upper tier low power amplifiers. This type of pairing is very dynamic and IMO more natural and real/live in it`s presentation.
The largest cause of speaker failure is having an underpowered amplifier that clips and distorts loud sounds which can occur in music without you even being aware of them. This can occur even if you aren't listening at reference level.
Some need it to break their lease.
Because we're trying to reproduce music, not just sound.

This is an bizarre concept to most of us, but work is the function of energy over time and when there is little time to do a lot of work, it takes a lot of energy.
"Because we're trying to reproduce music, not just sound"

Let me fix it for you:

We are trying to reproduce a stereo mix which boils down to multi channels such as guitar, vocals, drums, bass, keyboard, mic's that pic up room echo, etc. being processed into left and right. Then we obsess about how to fix things when the "images" are bunched up due to:
underpowered (chose your class and topology) amp, poor cables, poor transducer dispersion, lowly preamps, fill in the blank...

The concept of imaging w.r.t. the stereo mix is met with skepticism in my mind...sure I like "holographic" sound but it's all an illusion.
I also believe that this subject can be rather misleading to some. Sound pressure operates in a logarithmic scale. The difference between 1 and 10 watts is perceived as a 3db increase or a doubling of the volume. 10 to 100, then 100 to a 1000. So 500 watts is not "that" much more than 100. You cannot just say 500 watts is 5x more than 100. What it does allow is more current output and a better handling of steep impedance curves.
Thank all your for your responses! I got it know! Macd made a good point: it has to do with the logaritmic scale. Doubling the power does not mean to doble the perceived sound presurre level just a 3 Db increse. Thanks again.
You also have to keep in mind that a low powered amp pushing a hard load can clip because of the lack of head room. You may not even know your destroying the speaker beacause of soft clipping.