Why did Earth need material from comets to assemble the raw materials for life?
The earth in its early stages was far too hot for water to be in liquid form and the water vapor that would have been around at the time would have been blasted away via solar wind (http://en.wikipedia.org/wiki/Solar_wind). Anyone claiming to know exactly how water came to be on Earth is likely daft, but comets and asteroids seem to be the most common theory for the source. MinuteEarth, a YouTube creator has a great video about it but as a new user on this particular sub-site I cannot link multiple things at once.

https://www.youtube.com/watch?v=_LpgBvEPozk