Lots of people have work-sponsored or personal dental insurance plans that run from January 1-December 31. Your insurance benefits give you a set dollar amount for that entire year (e.g. $1500).
Take 2019 for example. If you haven't seen the dentist at all from January 1 until now, that means you have the full amount of insurance money left. If you don't use any of it before December 31, then it is gone for 2019, and unfortunately there is no way to get it back or use it in 2020.
So while you are paying for your benefits, it makes the most sense to come in for a check-up and cleaning while you have the room on your coverage. This way, if there are any fillings or any other treatment to be done, you can use up your 2019 coverage and still have plenty left when 2020 comes around and its time for your next cleaning and check up!