Aight so first, get good at grammar. That's literally the only prereq yet the average wn grammar is on par with mtl... I wish it was an exaggeration...
If your grammar isn't near native english level, then you have no business to be writing. Simple. If you want to write, get better grammar. (applies to all languages)
AND FOR THE LOVE OF PAPER, stop relying on softwares like Grammarly. Just writing, yeeting into grammarly, then posting is one of the main reasons why the quality is absolute garbage. And not even gold garbage.
Aight, now for the actual reasons, evidence, and all of those shenanigans...
When a reader reads your book, they're looking at your story through lenses. Grammar affects how blurry those lenses are, with the best grammar having crystal clear lenses.
Who cares if you get a chapter out each day if your readers have to write half the story for you to fill the holes left by grammar?
Poor grammar absolutely demolishes any sense of writing style that may be there too (examples later, ref: A1). Softwares like grammarly are checked by bots so it does the same thing, but it's not as apparent to a non-native speaker or someone who doesn't look for it.
And yes, it affects the reader's experience even if they're completely clueless. (examples later, ref: A2)
If I put the examples in here then organization gets funky and chapter becomes long so ye (the examples would be better of explained in the writing style section). "ref" means "reference" and the two characters after are just tags for labeling/recognition purposes.
Next bit: Following the trends will do absolutely nothing if your writing quality is abysmal. As in, absolutely horrendous. Cancer inducing. So terrible people wouldn't read 50 chaps even if they were paid.
Alright jokes aside, following the trends will only get you clicks on the first few chapters. If your writing quality doesn't at least have amazing grammar, then the only people who will stay are those without brain cells, people who are just using your novel to kill time, or very, very, loyal readers.
Only exceptions to this are novels with unique storylines or aspects, but at that point you're no longer copy and pasting the trends so ye.