The main reason for registering more domain names is to enhance your online identity by registering abbreviations, variations and positioning statements (slogans) that say something about your organisation, your products, your brand or even a special offer that you might be considering.
The key advantage of this approach is that you are effectively creating many different doorways for customers to walk in to your on-line business. By registering misspellings, plurals and hyphenated versions of your identity you are also ensuring that other businesses cannot hijack your identity or worse, your customers.
Another very important reason to register more than one domain, especially if that domain is your company name, is brand identity protection. If you want to seriously protect your brand, register the .com, .net, .org, .info, and .biz for the domain. Those are the main top level domains (TLD). The .com extension is the primary TLD. If you can get the .com domain, get it! The consequences of someone else getting the .com of your company name can be disastrous to the success of your online business.