If you are using Contact form in Drupal 8 and you ever got following error:

"You cannot send more than 5 messages in 1 hour. Try again later"

and if for some reason you want to increase that, then you will want to increase flood limit. However in Drupal 8 just like in Drupal 7 flood control variables are hidden, meaning you can't change them through UI. For Drupal 7 we had a nice Flood control module but it hasn't been ported to Drupal 8 yet.