What? You have no idea what an LCCN is?
LCCN = Library of Congress Control Number.
Read more here from the LoC website.
Also Mill City Press has a great description here.
Createspace info can be found here.
What this basically will be is the number assigned through a case that is opened by your publisher to get your book entered into the process of acceptance into the Library of Congress. This control number typically can be displayed on your copyright page if you were inclined to do so.
As I discovered recently with my first book, if you overlook this step as I had, you will need to open a case with the Library of Congress yourself and then send two copies of your work to them for consideration.
By acquiring a LCCN number prior to publication, your publisher – in my case Createspace – applies for you and also submits a copy of your book for consideration.
The difference in price initially on the surface is roughly ten dollars (US).
For a LCCN before publication through Createspace (at the time of this post) it is twenty-five dollars (US) while applying directly through the Library of Congress is thirty-five dollars (US) plus two copies of your book and postage to send them.
The Library of Congress, Washington, D.C.
While acquiring this number does not guarantee that your work will be accepted into the Library of Congress, it at least gets the work information entered into their database which other libraries will access.
Not all public libraries that you may have offered your book(s) will enter the information in all databases.
Is it worthwhile to consider as part of your publishing process? I would venture to say yes without question.
With every book that you publish there will be a budget of some sort for advertising and such and a LCCN number guarantees that it will be found in a place that makes it worthwhile.
Sometimes the best investment is one that although it involves a gamble of sorts does give a return worth the price and effort in the process.
So in the end, I will recommend it as something to seriously consider if not already place in that checklist that is used when you publish.
Every little bit helps 8)