Why is it recommended to have swappiness to zero?

Hi, I wanted to know why it is recommended to have swappinees to zero for all data and index nodes in couchbase?

To avoid swapping active node memory. By avoiding swapping active node memory performance should be more consistent. The node processes are typically long-lived (and are large memory consumers) so become likely candidates for swapping if enabled.

A description of swappiness, e.g. SwapFaq - Community Help Wiki should help make the recommendation obvious.