You don't need an account if what you really want to do is just read everything that gets posted on the site and leave comments on that material. (Your comments will be held for moderation to avoid any spam comments of course.) However, if you want to have a more active role in shaping the agenda of the site, you'll need to be logged in.
Here are some of the benefits of becoming a member of the site:
For one thing, accounts come with the ability to make and search profiles of other members. In other words, you can get to know each other better.
This is a collaborative space, and members have more say in setting the agenda. Members can create new forum topics, and can post "Take Action!" items, book reviews, polls and new web links.
If you are a member and you contribute regularly in a way that helps develop this community, you will be offered "regular contributor" status, which will get you a blog on the site, and the ability to post events and other kinds of updates, information, or newsy things on the front page.