There are probably as many reasons as there are participants.
Here are a few popular ones.
To do something with all this computing power
After finishing the RC5-56 and RC5-64 contests, it was a natural
next step because the existing client code base could be very easily
converted to run RC5-72. Instead of making a mad rush to push the next
generation, general purpose clients out the door, we decided to make
the small alterations necessary to allow the existing clients to
participate in the 72-bit contest as soon as possible. Providing new
client software quickly helped maintain the interest of our large
number of volunteers by providing a task we could continue to work
on as a group.
To prove that small-bitsize encryption is insufficient
A loosely organized group of people on the internet (us!) deciphered
an encrypted message in their spare time. That might be
sufficient reason for the governments, security firms, and others to rethink their current beliefs of what "safe" encryption bitsizes are. Decrypting progressively larger bitsizes
can only strengthen that argument by pushing the boundaries of what
key lengths are considered feasible.
To explore the feasibility of cooperative networked multiprocessing
It's fascinating to see how easy (and difficult) it has been to
band together this many computers, with this many operating systems,
in so many time zones and put them all to work on a common computing task.
Because it's fun
Perhaps in the same way that writing a tic-tac-toe program in
sendmail.cf is fun to some people, it's been a rewarding diversion
for everyone involved.
Because you can win money!
There is a 10,000 dollar (U.S.) prize for the winner of this contest.
After winning the RC5-56 contest we gave $1,000 to the owner of the
computer that actually found the winning key, we kept $1,000 for
ourselves and we donated the remaining $8,000 to
The prize distribution for this contest will be similar, and everyone
participating has a chance to vote on which nonprofit should receive
the majority of the funds. You can read more about the prize money
distribution on the
stats site. Other monetary prizes were similarly awarded for the
completion of RC5-64, CSC, DES-II-1, DES-II-2, and DES-III
To get to know more people
You'd be amazed at the number of people you can meet by participating
in this project, especially if you hang out in the #distributed channel on IRC,
or subscribe to the mailing lists.
To attract the opposite sex
Well, maybe it has, maybe it hasn't. You never know ;).