The way I learned it, swap is basically the inverse of file caching: in much the way that extra memory can be used to cache more frequently-used files, then evicted when a "better" use for that memory comes around; swap can be used to save rarely-used anonymous memory so that you can "evict" them when there are other things you'd rather have in memory, then pull them back into memory if they ever become relevant again.

Or to look at it from another perspective... it lets you reclaim unprovable memory leaks.