What real purpose would romantic relationships serve to provide that is not pointless fanservice for the reader?
Romantic relationships tend to have thematic implications. Hypothetically if Saitama and Tatsumaki were to end up together, it could be a great thematic victory in the story. While Fubuki represents power in a societal context (status/rank/social standing), Tatsumaki represents power at its purest and most primal, the kind of power that pushes others away. ONE has consistently shown that power isn't what it's all cracked up to be, and Saitama having already displayed genuine concern and relating to Tatsumaki is a great way to break through her walls and illustrate this, especially if they subsequently formed a relationship, showing that personal relationships trumps loneliness and power, a theme carried over from Mob Psycho 100.
No manga vs webcomic autism please. Both are great in their own ways.