In the (many) years I've been playing the piano (generally upright only), I've never felt the need to use the left pedal. From what I gather, the left pedal on an upright places the hammers closer to the strings, reducing the velocity of the hammer as it strikes the strings.
However, I (like to) think that I manage the dynamics of a piece perfectly well with just varying the attack of the keys - up to the softest of pianissimo (with the added advantage that I can vary each key individually). Is there any effect on an upright piano that cannot (easily) be achieved without use of the left pedal?
Sometimes when accompanying an unamplified singer I put my foot on the soft pedal and leave it there. This allows me to play with a little more verve without overwhelming the voice. In an ideal world I might reposition the piano, or drape it with something. But this isn't an ideal world...
It makes more difference on a grand, where the 'soft' pedal moves across to hit fewer strings, and other strings with softer felt on the hammer. A 'more ethereal sound'.
But your question concerns uprights, where what happens is the hammers are brought closer to the strings, so don't have the same terminal velocity available as when the pedal is untouched. This means that in ppp passages (that's not a typing stutter!), notes can be played more quietly with than without. Problem with trying to play very quietly is that occasionally, we don't press the key quite hard enough - result - no sound.
The left pedal is an integrated function of a piano. Fine if you can reach all the nuances of ppp and mp. I can‘t and there is no eager to me to achieve this without this help. So the answer to your question about existing effects is depending of the abilities of the pianist and his ideas and ideals.