I do think the history of one’s church is important. Every ritual and doctrine came from somewhere. It is important to know your roots. Even if the church no longer functions the same way, how it has behaved in the past is important.
I agree with the other responses, and would also point out that a history of religion, especially a cultural history of religion, reveals a great deal that an institutional history of a church might overlook. What interests me more than the history of popes and priests (though these are very interesting and legitimate topics for study) is the study of popular belief and religious practice. In many cases, there is a large gap between what the church teaches and what individuals often believe. There are deep cultural, social, economic, and other factors that have implications for human belief. There is actually a very large historiography of both church history and popular belief, in Europe and the Americas. I suspect the same is true of other places as well.
Yes, church history is important. All history is important! The church has been a major influence in pretty much every political, social and cultural institution of the west since Constantinople. Understanding world history means putting a lot of pieces together, and the church is a big piece!
For what purpose? Are you asking if it's important for the purpose of understanding society or for the purpose of understanding religion? I'd say, in either case, it is important. If one is trying to understand his/her own religion, Church history is important because it's important to know where that religion came from and how it started. From a societal perspecive, it's important because it tells us why we have done certain things the way we have in the past.
It is important at the very least from a historical and social point of view. Whether or not one is religious, it is impossible to deny that religion has had a huge impact on the history of many countries. Therefore, it is important to study church history.