Every Country has a Head of State and it's generally felt that it better if they are to some degree above the day to day politics required to run a country.
They "represent" the country at major occasions (good & bad) and are a symbol , as is the national flag and anthem. The UK has a hereditary Head of State - which removes a lot of arguments but it means it's a bit of a lucky dip as to the capabilities of the incumbent. In the case of Queen Elizabeth she'll have done the job as best she could for 70 years and that , surely, is worth remembering next year.
Sure her own family are a bit of a mess, and occasionally she hasn't reacted to events as many might have wished but she's never wavered, never been found doing dodgy deals with Lockheed, or living in palace in Bavaria, or playing the tables in Monaco while claiming to live a life of religious austerity.
The UK could have done a lot worse