This page on server hardening has a large section on adding a SWAP partition.
Why is adding a SWAP partition good for server hardening?
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It improves the availability of the system, which is also an important part of information security.
E.g. if there's a DoS attack on a memory intensive service, the box is less likely/takes longer to crash.
I agree with billc that the intention is to improve availability, however when most systems start swapping, performance drops so dramatically that you are facilitating a DOS for your users.
It is a good idea to be able to hibernate a system for fast recovery after a power outage (if you don't have enough UPS) which does require swap.
I'm struggling to imagine how the confidentiality issue described by Iszi would come about in practice.