There should be a page in the book describing exactly what each package does and why it's in the book. Each package does have a short description of its functionality, but it doesn't say what exactly you would use that program for or what other packages might depend on it. Here's a few descriptions to start with...
Coreutils - A number of essential programs for viewing and manipulating files and directories - not only useful to the user, but also needed by the vast majority of configure scripts for source-compiled packages.
Zlib - libraries for compression and decompression, used by a large number of programs (including a couple of LFS packages)
Vim - the creators of the LFS book believe that a text editor is absolutely essential to a basic, usable system, and vim just happened to be the one that they decided to use (maybe also have a link to the "Why is vim in the book?" thread that's linked in the FAQ, or summarize the contents of the thread here)
Ncurses - A library used by many other packages that manipulate text on the screen, and often use some kind of cursor or menu system. This includes: vim, bash, procps, psmisc, inetutils, texinfo, less, and other packages.
Texinfo - Provides the ability to view "info" pages, which provide information about programs and are generally more comprehensive than man pages. Also, the texinfo package itself (specifically the makeinfo and install-info programs) is needed to build and install info pages, which is why texinfo is installed in Chapter 5 of LFS.
Here's a few packages that I really don't use (directly at least - the only way I do use them if other programs run them) myself, and about all I know about them is that they're needed by other packages...
M4 Bison Flex Groff Mktemp (used mostly in various scripts)
