A better question would be: Why is it there in the Book of Abraham? Because really, it's not like it is very important for us to know about some star named Kolob "closest to the throne of God", whatever that exactly means. In my opinion it is not even clear what "closest" really means here.
If you read the chapter, you see that Abraham is shown the stars, but what God really wants to show him, what he is preparing to show him, is the pre-mortal world. The idea Abraham is getting from the stars that are shown to him is:
If there are two things, one greater than the other, then there is a third thing that is even greater, until we get to God himself. Likewise, if there are two spirits, one greater than the other, there is a third that is even greater, until we get to God. This is to show that even though spirits are eternal, there are differences and some are more righteous and "great" than others. And then he explains the plan that is set in place to lift his children up and help them progress.
Kolob is completely irrelevant in most other contexts, it is used as an example and symbol here. There is no really useful doctrine that can be taken from it, except in the context that it was used in. Although Abraham learned something about astronomy here, the real lesson taught is a spiritual one.