With swap: We can choose to swap out rarely-used anonymous memory that may only be used during a small part of the process lifecycle, allowing us to use this memory to improve cache hit rate, or do other optimisations. Without swap: We cannot swap out rarely-used anonymous memory, as it's locked in memory. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.