Generally at the time of registration most registrants register singular as well as plural version of a domain name if it’s available. And, also sometimes they register the .net and .org version of a domain name because after .com, .net and .org are regarded as the second best TLDs. Also, if it’s a two word domain name, a registrant may also chose to register the hyphen version of that name in order to keep full control on the name.
Well, it’s a good idea to register other variants of your domain name especially if you’re going to build a business on it and if it’s for investment purpose it’s not essentially needed.
There is no need to register too many variants of a domain name as it’s not needed but a recent look at verisignlabs gave me an entirely different picture of a possible domain registration for a press related keyword. Following were the domains:
The registrant came up with 14 variants to protect the 247-PressRelease.com which I think all were not needed except the 247-PresseRelease.com