On Feb 8, 7:31*pm, Nick wrote:
Because (large) prime numbers are used in all encryption systems.

If it's the 'largest known prime' it's probably a Mersenne prime
(2^n-1) in which case it's of no relevance to encryption, and would
explain its discoverer believing it to be of "no practical use". The
largest known prime has almost always been a Mersenne prime "since the
dawn of electronic computers":

http://en.wikipedia.org/wiki/Prime_number

Richard.
http://www.rtrussell.co.uk/