What does naming the chapters in a fiction book accomplish? Isn't easier to just number them instead of coming up with names?

It goes in and out of fashion.

During the 20th century, many ‘serious’ novels were published with chapters which were only numbered and not named, while detective, fantasy and other ‘light’ fiction often had chapter names, as did children’s books.

During the 19th century chapter names were fairly uncommon, but book cover or frontispiece illustrations were sometimes effectively illustrated guides to the entire book.

Medieval chapters rarely had titles.

These days, the notion of ‘serious’ and ‘light’ genres has largely been obliterated. What is serious (or not) is now mainly down to the commissioning policies of the publisher. For solidly commissioned genre fiction, you stick with whatever the publisher does. For author-submitted material, it comes much more down to what the author, editor and publisher decide together.

If you want to name them, name them. If you don’t, numbers will be fine.