Recurrence Interval - Last day of the month
There is no setting for setting recurrence interval for the last day of each month. There is only, 31st day, 30th day. However if you select 31st day it will only send on months that have a 31st day. So say you want to send an email to everyone on the last day of the month who made a purchase that same month, you would not be able to do that without a "last day of the month" option
Zoe Blatter commented
Also, could be used for other non-date specific items like "Last Friday" or "second Monday" of the month, or around holidays like "a week before Thanksgiving"