The Book lock date prevents modification prior to its date on a book. You would typically set a Book lock date the filing date of your tax return so no changes can be made prior to the corresponding files.

This give more control, specially when you are working with your team on the same book.
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​Set book lock date:

As soon as a Book lock date is set, modifications prior to that date are disabled, meaning that you can't change anything on this book before this date. A book editor is in the position to modify the Book lock date, enabling eventually necessary modifications.

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The "Closing date" helps you to enforce a closed period on a book.

Set book a close date:

If you set a "lock date" and "close date" in the same book, the transactions would be blocked to the most recent one. That way brings more flexibility and streamline the workflow with the lock date, as well as have more security with the closing date.

It's a matter of permission. Both works same way, by blocking any transaction from a date behind. The difference is that editors can remove or set the Lock Date backwards, as they wish, but only the book owner can remove or set the Closing Date backwards.

"Lock date" is more operational, to do intermediary locks and "Close book" is more strong, to really close a book from a date behind. So, when you want a book to be closed, and not even editors touch it, you use the closing date instead.