How does it influence where and how my book might be seen?
It is the first step to get visible to the readers at all. It is the 'genre' of your book.
Just look how you yourself by books. In a store the books are 'categorized' by aisles. If you want a book about Microsoft Word you go to the aisle with the sign "Computer". If there is one with the more specific category "Software" you would choose that one. Never ever would it come to your mind to look in the "Gardening" corner.
The same is true in an online shop (like Amazon). The categories/genres are the entry points to search new books (if you do not have an author or a title already in mind). Even if you do not click through the categories, you can define them in your search phrase.
So this meta information is one of the corner stones for the visibility of your book. It's the way books are categorized, it's the way people search for new books.