The collimating lens is necessary to prepare the laser beam for the focusing lens which gives it the sharpness and precision it needs to heat and read the disc accurately. When the beam leaves the laser source, it diverges (grows more diffuse as it leaves the source) and must pass through a collimating lens. The lens makes the beam parallel (that is, the edges of the beam do not diverge and the beam does not become diffuse over distance). The collimated beam is passed through a focusing lens, which draws the parallel beam to a fine point.
