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

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)@
http://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_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.

John