Uniwidth is not the same as monospace! Monospace is non-proportional, meaning each letter occupies the same amount of space. Uniwidth is proportionally spaced, meaning each letter occupies a different amount of space, but the space for each letter is the same across all font-weights and font-styles.
A uniwidth typeface allows using weights to indicate different label states – without altering the amount of space the label occupies inside of the layout. This is a valuable feature for functional text in screen design, like for headers and menu bars.
Lisa Staudingers article Uniwidth typefaces for interface design is an excellent guide into the topic. In addition, Lisa provides many links for uniwidth typefaces, like for example