Every once in a while I get asked by a client, if they should register multiple domain names. Maybe their own business name under .ca, .com, .org, .info plus theirproduct.com, theirservice.com and maybe even theircompetitor'sname.com.
The thing is, unless there is a website built under a domain there really is no Google-magic going to take place. So if there won't be any Google ranking... who will really knows it exists and in turn what value paying for it is there?
I am all for grabbing important variations of a website domain you own... like the .com of www.yourcompanyname.ca. I usually limit this to .com, .ca, .org.
You also may want to buy special promotional domains to use in offline ads... this way you can better track visitors from media like print flyers, tv or billboards.
Somtimes you might think of "cybersiting" on a domain... that is holding a domain that you think will have value to someone else at some time in the future. I guess this is possible but it is certainly a big gamble with all the options out there for someone to use names other than the one you own.
Maybe you think of developing a website under a popular variation. If you owned say... www.starbucks.ca (and Starbucks didn't) there may be some value to some mistaken traffic by someone typing the .ca in rather than the .com... but unless you have a killer website you won't get any search engine ranking... (and will only have random visitors of no business-value coming and leaving faster than you can say "Caramel Macchiato"). Plus Starbucks would probably sue you to get it... and win.
Truthfully, I have owned many domains for offbeat business reasons and I have yet to really have one that proves to have been a better investment than just working ("adding content on" and "building links to") my main domains.
Need to buy a domain? Using Google tools, I can help you buy the right one from the start.