Bookends: Why Bother?
It's pretty simple: The reason to use bookends is to keep your books upright and together.
For some, that's an answer to anal retentivism. For me, it's an answer to protecting my books both as an investment and for my personal demeanor.
Bookends Help Organize
Books are made to sit upright -- vertically straight on the bottom edge. The optimal way to store them is with even pressure on that bottom edge. The spine holds the rest of the book together, and when the book leans with uneven pressure on the spine because the book isn't sitting evenly on the bottom edge, the pages can pull away from the spine, especially when the glue dries and cracks or the staples or string suffer fatigue. Bookends, used properly, keep all the books straight with even pressure.
Is it proper to store a book vertically on its back edge (opposite the spine), such as when it's too tall to fit on its bottom edge? That would be a second choice, and better than other options. The problem with storing on the back edge is that it may allow the spine to sag or bulge, and if the book is wide it may cause fraying of the spine on the shelf above. Worse options would be laying books across the tops of shorter vertically stored books or leaned at an angle, all which may promote warping or sagging and stress on the spines.
Another good reason to use bookends is to group books by subject or other criteria, or even for aesthetic reasons. You may simply have a few books together in a large area and want to show them off or keep them from scattering.
When You Don't Use Bookends
If you're going to lean or cant a book, such as for display, make sure you use a tilted shelf with a lip that will evenly support the bottom of the book.
Certainly, it doesn't hurt to lay books horizontally on their covers. However, make sure that the pressure on the cover is evenly distributed to prevent warping. Furthermore, you don't want to introduce other objects on or near the book lying on its cover that might scratch or otherwise injure the cover. And, you should minimize the amount of weight you lay on top of books stored horizontally. Tabletop books are designed to lie flat on a table top or shelf, of course -- just make sure you don't stack more than a couple together, and keep the surface clean and free of obscructions.
Bookends Make a Great Gift
If you know a book lover or collector, or anyone else for whom books has a place in their lives, bookends make great gifts. Most book stores offer a variety, from the simple to the stylish.
(Bookends link above to Amazon.com through my online book store, through which I may receive commissions for sales.)