Let’s start with a vocabulary lesson:
A person who is Demisexual typically doesn’t experience sexual attraction to someone unless they have a strong bond with them first. On a similar note, a person who is Demiromantic typically won’t fall in love with a person without that same kind of strong bond. It’s on the same gradient as Asexuality.

“So you’re saying you don’t fall in love with a person unless you have a strong connection with them first. But that’s, like everybody. You’re not demiromantic, you’re just normal.”
Let’s get one thing straight: I’m not normal, I’m a writer. The two are mutually exclusive. But if I wasn’t a writer, yes. I’m ‘normal’. As are you, whether you’re demi, ace, allo, or anything else.
We’re the ones who quirk our eyebrows at Cinderella. “Wait, so you danced with him and you’re gonna get hitched? Are you crazy?”

I’d argue that alloromantics (people not on the ace spectrum)— also ‘normal’— are the ones who identify strongly with Cinderella. The ones who can have romantic chemistry within a single conversation, the ones who experience ‘a spark’ or believe in ‘love at first sight’, etc.
I didn’t do that. A lot of people don’t. But a lot of people do. And that is also okay.
“Well, of course you wouldn’t sleep with a complete stranger. You’re not demisexual, you’re [insert slut-shaming argument here].”
One of the things that tipped people off that I was demi was that I was just plain perplexed by things like hook-ups and pornography. The idea of hooking up with a complete stranger wasn’t sexy, it just seemed gross, no matter how I tried to wrap my mind around it.

That doesn’t make me a prude or ‘smart’ or whatever, it’s just not how sexual attraction works for me. You find the opposite in hookup culture, etc (I’ll admit, I’m not hugely knowledgeable on this one because it’s a personal turn-off), where people are perfectly happy being physically intimate with someone they barely know. That doesn’t make their way of getting sexy any better or worse than mine, they’re just different.
So why does it matter?
I personally believe that a decently large percentage of the population falls on the gradient of demisexual or demiromantic, but just don’t recognize it as that label. I believe that recognizing it as distinct from its opposite can make it easier to understand and appreciate the differences between them.
It can help us to recognize people who feel love at first sight as alloromantic, rather than claiming they’re ‘immature’ or ‘flighty’. It can help us identify a person unwilling to join the latest dating app craze as demisexual, rather than ‘unadventurous’ or ‘prudish’— and recognize the people who do enjoy it as allosexual, rather than ‘slutty’.
Having a name for it often makes it so much easier to accept that they’re just different flavors of love.
And they’re all awesome.
But what does it have to do with writing?
As you write, you might find yourself condemning one particular sexual or romantic orientation without realizing it, using a lot of the same misconceptions that I mentioned above, and that can be damaging and alienating to readers who belong to that category– and alienating readers isn’t something most writers strive for.
Differences in orientation (and misconceptions about both) can also be sources of legitimate conflict between characters, who can butt heads about what is and isn’t appropriate in a relationship.
You can also build up conflict between romantic partners by having one member be demi while the other is allo. It isn’t stated explicitly (it rarely is), but I would make a strong argument that The Hunger Games’ Katniss is demi, while Peeta can easily love her from afar. The same goes for Welcome To Night Vale‘s Cecil Palmer– who falls in love instantly– and Carlos the Scientist, who needed that bond beforehand. In both cases, you’ve got one character who’s stuck dodging the pining affections of the other.