Okay, not to kick the hornet's nest on my professional blog, but please believe me when I say that 90% of the authors who advertise their works this way are annoyed by using-AO3-tags-in-advertising at best and actively hostile to the concept at worst. You have to do it anyway, whether you want to or not, especially if you're writing romance, YA SFF, or (gods help you) YA SFF romance.
traditionally published authors hate writing summaries just as much as fic authors do. how do you sum up your giant magnum opus in the space of a tweet? summing up your magnum opus in five (5) ao3 tags is even worse. No one wants to do it. even romance writers (who often write with AO3 tags in mind) don't want to do it.
so why the heck are we doing it, if it sucks so much? well.
AO3 has by no means killed the traditional novel, but it has kneecapped the market for midlist genre fiction aimed at young women. If you want a cozy comfort read you don't have to think too hard about, are you going to spend your hard-earned cash on a brand new story you may or may not like? Or are you going to go to AO3 and read about Blorbo and Scrungly getting together again?
if you want to become a professional author, especially if you want to become a professional author of The Kind Of Stuff That People Do Fandom About, you're competing with infinite quantities of Blorbo and Scrungly. there is no way to comfortably make a living as a midlist SFF author for young women anymore without doing something to stand out amongst the Blorbos and Scrunglies. you've basically got three options at this point.
Option one: Grow up in fandom. Write good fic. Organically grow a friendcircle and a fanbase. When you get traditionally published, people in fandom already know you, know your fic is good, and know that you are a person they'd be willing to pay money to see happy and succeed. They might be willing to spend some money on your original IP- to make your stuff their fandom- because they've already been fanning over it for years.
Option two: Write something so weird and evocative that it will get people's attention just because it's so freakin' weird and evocative. We're talking "lesbian necromancers in space". We're talking "murder mystery in a halfway house for Portal Fantasy survivors". We're talking "Hamlet in a vaguely West African milieu, but the story's being told by a space god that's shaped like a giant boulder and is using the mortal intrigues to her own ends".
If neither of these options are open to you- if, say, you don't have the imagination to write something batcrap wild, and you don't have the fandom friend circle in place to help support you- you have to try to get Fandom People's Eyes On You in whatever way you can. because if you're writing YA, fandom is your target audience, and fandom is the audience who will make you or break you.
you have to convince fandom people, whether it's booktwt people or booktok people or bookblr people, that your book is worth, not only reading, but shelling out money for. and thus far? no one's found a good way to do that. if you're an indie writer, you can at least avoid making a fool of yourself in the process, but if you're going traditional? your publisher is going to insist that you do things to market yourself to fandom.
....there's a reason I've chosen to indie publish my adult fiction and make it pay-what-you-want. I can't compete with AO3. No one can compete with AO3. but because a handful of easily-fandomizable YA/adult books have blown up in the last decade- usually, easily-fandomizable books by authors who already had a fandom presence- the publishing industry as a whole hasn't figured it out yet.
so the ao3 tags trend, and the book aesthetics trend, and the MAKE A FANCAST trend, and every other publishing trend that looks like it's designed to pander to fandom? they're all absolutely designed to pander to fandom. we all know this. we all hate it. we all know that it doesn't work, that we're making a fool of ourselves, that we're actively discouraging a huge chunk of our target audience from reading our stuff.
i guarantee you most of the writers who are doing this on insta or booktok or wherever are going offline and screaming about how much they hate having to do this. i double guarantee you the writers who are doing it enthusiastically and don't get why it's dumb and pandering are going to regret it later.