There's a few reasons why registering is handy.
The data is released into the public domain, with registered users submissions being attributed to them. When you register, you can set who you want to be known as once, rather than typing it each time.There are a few other benefits to registering, such as:
No, not if you don't want to.
Data can still be submitted to, and downloaded from, the project if you don't register. There are a few features available to registered users that aren't available to unregistered users, see above under the section 'Why register?' for more information on this.