I think I'm going to have a Bernhard moment here:
"The una corda pedal on fortepianos"
The "soft" pedal actually does two things, besides softening the sound it also changes the colour of sound. The reason why it's the "soft" pedal is because depressing it shifts the hammers and keyboard (keyboard only on grands) to the right, so that only 1/3 strings are hit. It was first referred to as the Vershiebung (shifting) or una corda (one string) mechanism. The first una corda pedal was operated by a knee lever, then replaced by a pedal near the 1800's. By the late 18th and early 19th century, it was possible to depress the pedal gradually in a way that would shift the mechanism from three strings (tre corda) to two strings (due corde) and then to one string (una corda).
On modern uprights, instead of shifting the keyboard, it moves the hammers closer to the strings. This shorter distance results in less acceleration of the gammer, thereby reducing the force with which it hits the string and consequently, the volume of the tone.
Ok! Now to your post

:
Soft pedal markings begin to appear as early as Beethoven's time. Because the soft pedal is a basic component of the piano's possibilities, it should be used whenever the pianist finds its unique sound desirable. Keep in mind that while the soft pedal does help soften the dynamics, its main purpose is to change the colour of the sound, just as a mute does on a violin or trumpet. The soft pedal can be used at any dynamic level,, not only piano or pianissimo. E.g. in
Serenade of the Doll Debussy writes "
Il faudra metre la pedale sourde pendant toute la duree de ce morcea, meme aux endroits marques d'un f" (use the soft pedal throughout the piece, even where marked
forte.
Pianists should be able to produce a soft sound using only finger technique, you shouldn't automatically apply the soft pedal everytime the music calls for a soft dynamic (like Pollini). BUT! pianists should not revel in the excellence of their dynamic control to the extent that they find it a virtue never to use the soft pedal. This would deprive their interpretations of an excellent resource! (*COUGH! Ravel).
The soft pedal should only be used:
1. To create a new "colour"
2. To aid in contrast for a subito pp
3. To aid a diminuendo (while being added gradually)
4. To create a resolution (by adding it on the final note, or chord)
5. To create special effects (while playing mezzo forte, forte etc.)