Spear Dancing Reach = While using Spear Dancing Style and making
a full attack using the chosen weapon as a double weapon,
you can grant the reach special weapon feature to one or
both of the weapon’s ends until the end of your turn as a
swift action.
If I'd use a weapon like a horsechopper which already has reach, would I still need the feat?
Yes, because you're using Spear Dancing Style.
Benefit: Choose one weapon from the polearm or spear fighter weapon groups. While using this style, you grant the chosen weapon the double special weapon feature, using the weapon’s normal statistics for its main-hand end and the statistics of a light mace for its off-hand end. A weapon wielded in this way loses the brace and reach special weapon features.
missed that, thank you!