Experience with LCCN's?

In preparing the front matter for my historical fiction book, I’ve come across the LCCN, as well as some conflicting information about its use, and it’s left me with some questions.

1. It seems like not all LCCN requests are accepted. Is this correct? And is there something I can do to have a better chance of being accepted?

1. Is the only purpose of an LCCN for cataloging in the Library of Congress? Does it also make your book available to libraries and if so, is this the only way to make it available to libraries?

1. Is there a reason why I wouldn’t want an LCCN?

For those of you who’ve had experience with this, I sure would appreciate your thoughts.

Anna