Nuts in a cheesecake recipe add their own unique buttery flavor. They're just another of the many different ingredients you can add to your cheesecake to create a new and unique taste.
Plus, nuts are a great source for monounsaturated fat, i.e. the good fat, and they have no trans fat whatsoever. Hooray!! 
Since most of us don't want to sit around cracking nuts all day, you should buy nuts that are as close to natural as possible. That means unsalted and with as few additives as possible.
When using them in your crust, use a food processor to chop them using on-off pulses from about 5-10 seconds. When finished, the nuts should be the size of sea salt.
For the cheesecake filling chop the nuts in the same manner as the prceeding, but for less the total time. When finished the nuts should be about a 1/8 to 1/4 inch in length.