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?