Grand pianos: The soft pedal is used to change the mood of the sound from percussive to more serene and gentle when the soft pedal is depressed. It should not be used solely for producing a softer sound because it will also change the timbre. In order to play pianissimo, you will just have to learn how to play more softly. You can produce very loud sounds with the soft pedal depressed. One difficulty with the soft pedal is that it (una corda, or more correctly due corda for the modern grand) is often not indicated, so the decision to use it is often left to the pianist. For uprights, it mostly decreases the volume of the sound. The soft pedal on most uprights has a negligible effect on timbre. Unlike the grands, the uprights cannot produce loud sounds with the soft pedal depressed.

Many pianists do not understand the importance of proper hammer voicing for the soft pedal to be effective.

 

http://www.pianofundamentals.com/book/en/1.II.24