To the Monarchists, why does Nepal have to be a Hindu Rashtra if Nepal were to be a Monarchy?

I do understand that the title of monarchy comes along with a religious mandate of sorts. But does that mean a state religion has to be declared? For example in Thailand,' While there is no official state religion, the constitution requires the King to be Buddhist and declares that he is the “upholder of religions”.' This is quoted from the US state government website.
The example I have given is just a modern example. There have been other monarchies in the past without state religion or a religious background like the Kingdom Of Albania.

What would be the benefits of making Nepal a Hindu Rashtra?