First of all, let's suppose that you have a primary website that sells products that are very well-known. You may want people to be able to reach that site by typing in the name of the product, rather than having to remember the make (the brand or company name that makes the product).

A good example of such a situation is Microsoft Windows XP; if someone wants to look up info on Windows XP they may not think to go to Microsoft's main website. They may want to reach it by trying to type Windowsxp.com into their address bar. Microsoft is, of course, aware of this fact (thanks to clever marketing personnel) and has already set up some of the obvious possible web addresses to redirect back to their main site.

Redirects are not the only reason to purchase multiple domain names though. Another interesting angle is to buy up domain names that you predict might be useful to other businesses, and then re-sell them at a higher rate when someone needs to purchase that domain. The ethics of this are arguable.

Another reason to purchase multiple domain names is because you would like to run multiple websites. You may have a company with different branches. Perhaps you have more than one company you are running! Or even still, maybe some (or all) of the websites you want to create are more informational or just for personal use (for example, to show artwork or photography to friends and family).

Purchasing multiple domain names is useful for a variety of reasons, and it just depends on your particular situation.