I don't get why this woman is always too much convinced of the trivial things she's doing in a way that makes her feel sooo important.
Because she’s a writer, and because her current work is all about these trivial things. Of course they’d be important, it’s literally how she pays her bills and puts food on the table.
EDIT:
That was just the in-universe reason.
Out-of-universe, she's the protagonist of
Haruka Reset, which is literally named after her. Therefore, she's the single most important character in this manga. You could remove any other character and it wouldn't affect this story in any way. Remove Haruka, and you might as well just go read something else.