Imagine you have a view that lists upcoming events on your Drupal 8 site. There's a date filter that filters out any event who's start date is less than the current date. This works great until you realize that the output of the view will be cached in one or many places (dynamic page cache, internal page cache, varnish, etc). Once it's cached, views doesn't execute the query and can't compare the date to the current time, so you may get older events sticking around.