The purpose of knife edging is to align more than one beam as close to another so that they run parallel. Depending on your beam size this will enable you to either use telescopics to reduce the beam down into a usable beam or use lots of small beams as one beam.

Heres an example of knife edging -

Heres an example of where you might use knife edging -

To knife edge beams you need adjustable mirror mounts and mirrors. You need adjustable mirror mounts so you can align the beams perfectly -

Beams need to be aligned to make as squarer/rounder final beam as possible.
So say you are aligning 6 beams, if your beams are 3mm wide by 5mm high at aperture then you need 3 across and 2 down. This will give you a joint beam size of 9mm wide by 10mm high as shown below.
If you were to align all your beam in a line you would get a joint beam of 18mm wide by 5 mm high, this is not a good shape beam.

For the above example you would need telescopics to shrink the beam down into a usable beam. But if you were to use different lens on each laser then telescopics would not be needed. If the lens was to give a 1mm wide by 1.5mm high beam at aperture then arranging the beams the same as the above 3 across and 2 down you would get a beam of 3mm wide by 3mm high this could then be used in a projector.

The tighter the beams are aligned the better the quality of the final beam.

So to conclude this page, the purpose of knife edging is to align a set of beams parallel to each other. You will then have x times the amount of power in a single usable beam.