Nope, they are not necessary. I think it is more a family meeting. If I am not wrong Henri and Philippe are cousins, and the countries are so very close to each other.
The question is why they are making an official "state visit" about this, a bit strange IMO. Maybe thats why MT looked so bored listening to the national anthem.
Yes GD Henri and King Phillipe are first cousins. Henri's late mother Josephine-Charlotte and Phillipe's father Albert were siblings.
Also both recently celebrated birthdays! Here's photos of the cousins blowing out candles on a joint birthday cake.
https://www.instagram.com...mX/?hl=en&img_index=1