This is worth a read. It's an article explaining the reason behind the need for certain PS3 games to install to the hard drive instead of being read directly from the disc. It's all down to the way a Blur-ray head reads the disc itself, apparently, which is different to the way a DVD drive works.
A Blu-ray head can only read at one speed, while a DVD head can read at multiple speeds. Discs, obviously, move faster away from the centre as they spin. A Blu-ray drive has problems coping with this, hence longer load times if a Blu-ray disc is treated by a developer in the same way as a DVD. Installs are a way of beating the "problem".
Of course an install isn't necessary. It's just easier for the people making the game. Read the whole thing for yourself.