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Hi Antony,
The object of collimation on your Dob is to make sure all the light coming from the primary mirror reaches the centre of the eyepiece. To get there, all this light must be reflected by the secondary mirror. If the primary mirror is tilted off-axis, some (or even all) of the light could miss the secondary mirror i.e. pass back out of the tube. If the focal point of the primary mirror is off to the side in the eyepiece, you will get large distortions. If the focal point is way off the eyepiece altogether, you won't see anything.
The laser helps to align the secondary mirror with the primary mirror. Then by returning the beam to the centre of the collimator you put the focal point of the primary mirror in the centre of the eyepiece.
See attached diagrams.
Regards, Rob
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