Brand conservation is a reason to buy the various extensions of a domain name, so if you feel that there is a possibility that someone would want the same domain name with those extensions, then this might be a good idea.
Also, if you have multiple domains for the same site, say you have mysite.com and mysite.org, you can publish the site to both of the domains so both will work. You would then make one the primary. Here is a link with more information on how to achieve this: Assigning domains and creating a primary domain.