It's subtle, but I do think their support does a good job of setting up how their viewpoints differ on devotion, Hubert as cold and ruthless yet utterly selfless while Dorothea is warm and kind yet still fundamentally self-centered
I see these supports mostly dragged up to debate whether Edelbert is canon or not which I think is a disservice to the actual contents, Dorothea explicitly joined the Garrag Mach to find a husband that would provide her financial security, and so she defaults to this romantic lens when viewing intimacy, there's a great line she has about how "loving another is really about wanting to be loved," the idea of being compelled to submit your life to someone else without asking for anything in return is something she can't fathom because she's never been able to afford it
And it is pretty funny she jumps to marriage after that, but I think it shows how even when trying to mirror that same devotion she's seen in anyone else, she still needs a safety net of sorts, and the fact in their paired ending they don't marry despite this speaks of it being a deliberate character action rather than typical romance convention
To me their paired ending really speaks to a path where depressed, pacifistic Dorothea finds a way to thread the needle between her aversion to violence and her agreement with Edelgard's ideals, and while Hubert really remains unchanged by the support chain, I do think Dorothea uniquely actually tries to understand, in most of the other supports he's the one to give into others which I tend to think on their own are fine/well-suited, but the fact he gets the gain a companion in the shadows is something I think he might not even realize he needs
Plus on a very iddy level, sexy spies are great