Why would it matter if they're friends? It won't make any difference whatsoever to the relations between the two countries since royal families no longer play a role in policy making.
I will never understand the obsession of some on this website with the royals being 'friends' with one another. It's entirely irrelevant. The point of international visits for royals is not so that they meet their old friends, it's to promote the UK abroad, nothing more, nothing less.
I'm struggling to see much of a point to this visit in all honesty.
I'm one of those who believe it's a good idea if royals create personal relations, ideally friendships.
I disagree with you in regards to royals being without influence. Depending on the country and traditions as well as the role of the royal families and especially
depending on the subject, they do have varying degrees of influence. - Certainly in regards to the public.
Royals are supposed to altruistic and royals from different countries often cover roughly the same topics. It's IMO always a good idea to exchange ideas and visions and experience in order to get the message across to both a domestic as well as international audience.
Let's take landmines. I doubt many western politicians would tell their royals to mind their own business on that one! If one royal address the issue, it's good. If two or even more talks against landmines, it's even better and reach a wider audience. Meaning they can benefit from each other and support each other.
We can discuss how much difference one royal speaking up against landmines make, but two or more prominent royals, then it begins to matter. - It's all a question of PR.
It's easier to co-operate and co-ordinate if they know each other personally.
As for personal relations when visiting each other, like on this visit.
Like with everybody else, royals too tend to be more relaxed if they know each other, and even more if they are friends. And that a totally different atmosphere which again sell better.
Here it's clear that V&D and W&K don't know each other. They are friendly, absolutely, but also a bit stiff.
Compare that to the interaction when V&D visit or are having a visit from royals they know and are friends with. It's a much more relaxed atmosphere, where they chat and joke and often forget they are on the job. That's a relationship V&D have with pretty much all other European royals of their age range.
The purpose of visiting Sweden? Showing the flag of course, but also creating a positive image of Britain, certainly in connection with the Brexit negotiations. The public in the smaller countries close to Britain, have a good deal of sympathy for Britain and most don't want to see Britain getting too hard a time at the negotiations. In comes a young likable couple; W&K.
They may not make much of a difference, but a little good PR is better than none IMO.
Apart from that, it's of course a practice run for W&K, for future state visits. Sweden and Norway are a safe, friendly and uncontroversial environment . You only become good at something if you do it fairly often.
And finally my pet-argument:
Royals are in a unique position, with very, very few who genuinely understand how it is to be a royal. It is IMO a good idea if they create personal relations with people in a similar situation, in order to learn from each other and exchange ideas and experience.
I don't think petticoat was referring to just you. Yours was pretty tame. But there have been posts which have suggested things like 'bringing George and Charlotte so they can start having playdates'. While its always great to build relationships between royal houses, 'friends' might be a push. Victoria, and as an extension her husband, have a close relationship with Haakon and Fred as she has basically grown up with them. Its a long established relationship. The continental royals interact with each other on a private level. We see the Norweigans and Swedes on private vacations. This isn't a level that's going to be established with the Brits in the near future if ever. The British royals have been isolationist for many decades now. Edward and Sophie come closest, having established bonds with the houses having attended the events on the continent for years. 
