At the university level, professors generally have a fair amount of discretion as to what textbooks to offer, and there do not generally exist detailed curriculum standards. Even here, though, if one wishes to produce a textbook for an Introductory German course, one ought to follow more or less the sorts of things that professors will expect to find covered there.
But at the grammar school level, adherence to standards is of crucial importance. If a book does not adhere to standards, then school teachers MAY NOT use it. Such work will then be a wasted effort in large part.
There can of course be reasoned exceptions to the idea of writing to standards, but in general, it is a very good idea. As it turns out, it is also quite helpful to generating and sustaining some consensus about what a book is to contain.
What should a high school chemistry book cover? Well, we could argue until the end of time about what would be a good idea from a philosophical point of view, and it would not be harmful to have several different variants. But at the same time, if the goal is to make something that people will find actually useful, then adherence to various official standards for what belongs in such a book is a great idea.
California and Florida publish very detailed standards. I am sure many other US states do as well. In the UK, I believe there are similar published standards. I would assume that more or less the same thing is true in almost all languages.
--Jimbo