"Cache dates" details: when to cache, when not to?

Issue #1439750: Incredibly slow loading times, hammering the database. mentions the "Cache dates" option in a date field. I hadn't noticed this before, and haven't found a more detailed writup. The explanatory text is "Date objects can be created and cached as date fields are loaded rather than when they are displayed to improve performance."

I'm trying to understand exactly what this means. Does this mean that if a date is fetched multiple times on a page display, time will be saved? Is the caching only for a single page display, or does it persist across multiple page requests? When might it be unwise to leave "Cache dates" turned on?

Thanks for any further explication.