You should register a Domain Name for a number of reasons:

They are scarce.

Internet Domain Names are a rapidly diminishing commodity. They are allotted on a first-come first-serve basis.

They are important.

The Internet Domain Name has become a vital part of corporate identity.

In the 21st Century, your Domain Name will be as important as your trademark and your presence in a global marketplace.

They are versatile.

For example, any email addressed to you at ceo@yourcompany.com can be forwarded to your current email address. This will prevent the need to reprint stationary if you decide to change Internet Service Providers.