Useful reasons to purchase an additional domain:
Email Domain – Your additional domain will need to be different than your primary website address, and so it can be shortened and used as your email domain. For example: if your domain is 20 characters long, this may be difficult and confusing for an email address. You then have the option of registering a domain with less characters (perhaps an abbreviation of your primary domain), which will allow for shorter, easier to remember and less confusing email address (i.e. http://thisisaverylongdomain.com – Primary website domain > http://thisdomain.com – Secondary email domain).
Domain Misspelling – If your domain is misspelled often by your target audience, it is a good idea to register a second domain in the misspelled version which will forward users to the correct domain version. Example: You have the domain http://accommodate.com registered, but people often enter http://acomodate.com into their browsers. It will be beneficial to purposely register the http://acomodate.com domain which can then forward site visitors to the correct domain. People will then soon realise their mistake. Hopefully.