A Battle.net account (https://eu.battle.net/) allows you to access the Blizzard Store and sign up for beta tests, and is a good way of keeping all your Blizzard game accounts/keys in one place. Merging multiple World of Warcraft accounts into one Battle.net account also allows you to keep a single easily remembered login name for all of them. There will also be additional features in the future which require a Battle.net account.

However, there are a couple of things to keep in mind when creating your Battle.net account:

• Make sure to create the account in the region where you usually play
  This is to avoid downtime of another region's login servers from affecting your access to the Battle.net account and thus your game accounts. For most of you reading this, Europe will be that region, so make absolutely sure you go to https://eu.battle.net/ when you register.
• Remember your secret question
  You will need it someday. This is probably especially true if you think you won't. ;)
• Think about account security
  You can find security information and tips at the following link, and using an authenticator (or the mobile authenticator) is especially recommended: http://eu.blizzard.com/support/article.xml?locale=enGB&articleId=36070

For questions about Battle.net accounts, please have a look at the Battle.net FAQ:
http://eu.battle.net/faq/index.html