Support for BitTorrent protocol enhancements, including better tracker support and peer exchange (PEX), which helps in finding and connecting to more peers for efficient downloading.
µTorrent 0.9 never existed (versions started around 1.x). If you mean an extremely old build (e.g., 1.9 or 2.0), modern OSes may not run it well, and it has known remote exploits. utorrent09 better