<productname>.com is considered the most important domain name to have, the reasoning being that when people hear of product X, they'll go check X.com

Dropbox started on getdropbox.com, but they found out lots of people reflexively typed dropbox.com and saw a squatted domain. So later, they bought dropbox.com

I tend not to agree with this 100%, I think most people google the name of the product, instead of trying X.com directly.