Why do you want DivX Pro? Because it’s free! At least for today, DivX is offering a free download of DivX Pro for Mac users, normally $19.99.
DivX Pro gives you the DivX Converter and DivX Pro Codec. Unless you are making videos there is no reason for you download this, even though it’s free. If you are a video maker editing on an Apple have at it. The Pro version gives you a high end codec allowing you to tweak everything in the encoding process from bitrate to quantization. The Converter is useful for converting large video files from whatever format to DivX with extra control and ability to batch conversions.