more often than not, i adopt the tarantino approach to taboo/violence in my stories: it's some extra flavour to the story. that doesn't mean I won't try to incorporate it with some thematic/plot elements too, like tones of depravity or to display that one guy REALLY doesn't like the other guy. I'll try my best with the latter, but it's usually to strengthen the former. it's "come up with the scene, then try to connect it after" for me. that goes for ANYTHING taboo, though I usually try to avoid rape and wars. the former because I don't like and don't feel competent enough to depict it to its full extent and the latter because I am a strategic and political numbskull and that the extent of my knowledge in warfare is "castle = instant loss" and "if you have double the troops, surround them", and I'm not even sure if those are correct.
as for romance, I like to start with the idea of character deprivation; that's not a real term, by the way, I made it up. the idea is that love is about filling holes, metaphorically and literally. you love someone because they complete you as a person, or that their companionship takes you to a whole new world you want to stay in. with that in mind, I write my romance around that concept. say, for example, there's a homeless guy who lives a life of vagrancy. he's essentially a vagabond. then you have a girl who's very sheltered and well-taught, but she feels trapped and feels as if she's never once been in a situation where she can fully realize her potential. now that I've constructed two puzzle pieces, all I need to do is connect one another, and let the rest build itself. is the girl truly ready for the world? would the homeless guy be used to a sedentary, domestic lifestyle? now you have a strong base, all you need to do is construct scenarios where these questions would be challenged, along with their chemistry.
as for purpose, it's optional. you don't need EVERYTHING to connect to the main theme of the story. sometimes you can have a mini-arc or short story happening in tandem with the main plot. try to attach everything to the through-line, and you'll get a convoluted mess of themes and tones that MIGHT clash against one another if you have too many. not all threads are born equal. got a nice idea you think might fit into your story? don't overplan. write the damn thing out first, then go from there. this might not sit well with the planner type writers but sometimes, spontaneity is all you need to find a potential breakthrough.