The system caches a site's content to make sure the user experience is a smooth one and the default setting is suitable for most sites and need not be changed. However, a cache update function is built into the UI to allow you to to force an update if significant changes to content have been made and to see changes reflect in the site variants quickly. This function can be accessed in Settings > Site Caching > Edit > Clear Cache > Now > Yes > Update.
The publisher can chose how frequently the cache is refreshed by going to the UI and adjusting the time 'Max Cache Time' but this will slow down the site so it is recommended this is only done when major, frequent changes to content are being made. The default setting will be suitable for most of the sites most of the time and is the best balance between cache refresh and system/site speed.