In certain VN's, primarily those that already have good storytelling, I think the H-scenes are their own form of distinct characterization. Before everyone looks at me funny ---
Everyone has different tastes when it comes to sex. Everyone puts a different emphasis on the various aspects of it, which is almost like a person's own signature. Is this person a passionate lover? Are they more of a giver than a taker? Are they rough? Do they like extended foreplay? Do they have habits ( like random bitting )?
Realistically, people are complex and deep. A person who seems gentle on the outside could be an entirely different sort of beast once the clothes come off.
VN's are similar, yet obviously not. We see the public( and sometimes private-but-not-sex-private )side of these character's traits -- their mannerisms, they hopes, dreams, their love(s) - and that's the 'normal scenes' for most VN's. Then we get ... a few ( sometimse a lot ) of panels of an h-scene.
An h-scene, in GOOD VN's, is usually just the act of expanding upon that and showing another side to that character. Even the position can be demonstrative of a person's personality - even more so is how 'attentive' those characters are afterwords. We only get so long of reading an h-scene before it ends, so the different positions, different things said, different angles, are usually a way to convey that both didn't just...do their business and call it done ( although: I have seen a few VN's where that's all the h-scenes, early on, were. Quick, to the point, and very loveless. But they were meant to be. ) There was more INVOLVED there, one way or another.
On the flip side to this, a few VN's tend to edge against that and have progression in their h-scene's. An early h-scene could just be...rough, flippant, distant. "Casual", if you will. But as the story progresses, the h-scene's become more involved, more loving, more tender ( or the opposite - start out loving and turn distant, but I have only seen that ONCE, and it was in an Otome ). As that character 'grows' and experiences things with their partner(s), the h-scene's themselves change. There's a few instances where I've seen this ( a VN with more than a few h-scene's -- at first I thought it was just going to be pure, all out h-scene after h-scene, but it actually had a good story, and each h-scene depicted some measure of 'trust' and 'growth' in the characters).
The entire premise of the tsundere-archetype is built upon that same principle: a character who could initially start out cold or hostile, then warm up over time. And 'giving yourself' to someone else is one of the big acts of "trust" ( I say thise loosely because that definition changes based on each individual ) that generally comes with life, and would it really be the same if you're just TOLD how much this girl loves this guy -- rather than see it? If there wasn't ANY build up, if there was NO sexual tension, but these two characters 'loved' each other, would it "feel" right? Would it convey that same sense of emotion, trust, and connection? In some stories: it can. And does it well. But in many, it's the 'it finally happened' moment. Especially in stories that don't have an antagonist or challenge to overcome ( mostly romcom's -- which makes it hard for me to read them lately ).
The opposite question also rings true. If these two characters constantly eyed each other, held hands, kissed, came -so- close to going the distance...but never did, would it feel REAL? If there was never that moment of, "THEY FINALLY GOT TOGETHER" after dating for two years -- would it cheapen the entire experience? Going through a 40 hour playthrough of a VN, watching these two characters almost lose each other over and over, struggle to be together, struggle just to survive, or struggle to remain faithful to each other, and finally get a moment to themselves -- wouldn't it be MORE strange that nothing happened between them?
( Note an exception: there are VN's where characters, male and female, do everything with a pulse -- which is, in and of itself, a statement of that character's personality. Maybe a traumatic past that they just want to 'block out'? Maybe they just enjoy it? Maybe the guy likes to 'conquer girls', which is yet another personality example? Or the opposite -- a woman who 'conquers men'. )
tl;dr:
Sometimes it's easy to tell which h-scene's are just "in there" for the sake of being in there, and which are truly expanding on the characters stories. Sometimes it's not. Usually good VN's are backed with good h-scene's that have some deeper purpose than just being 'thrown in sex'.