There a lot of reasons that's why it is necessary to treat commercial TiO2:
1. In many of the cases, titanium dioxide have impurities that are necessary to remove because could affect catalytic activity.
2. Do not have a high surface area. Commercial TiO2 only have at least 34 m2/g, which could decrease considerably the reaction rate of your material in a specific reaction.
3. Commercial TiO2 have a very specific "shape" or "architecture" that could do not favor your reaction.