LeatherUI simply replaces the white marble texture from the campaign and battle UI panel with a leather texture used in other windows. This makes it easier to see which unit is selected. Also makes you look pimpin'.
Makes it nicer to look at and makes selecting troops easier, though it does make seeing recruitment slots slightly harder - 9/10 for almost perfect mod!
P.S. I think toombs1234 is just trying to be a d**k