Pikipedia:Why create an account?
Creating an account is free and easy. It also has several other benefits to editing while logged out.
Anonymity under your chosen user name
Although those editing while logged out are often called "anons" on wikis, they are actually identified by IP address. This is visible to the public in the edited page's history and in recent changes each time they edit. For some editors, this doesn't give any information to others. But in some cases, such as if you are editing from a work computer, this may allow others to work out your location or other details. If you are worried about this, you should log in to edit. Your IP address will then not be visible to other contributors.
A way of joining the community
Becoming known under a user name means that you will be able to gain reputation and recognition in the community. It's difficult for someone to remember whether or not they've met "184.108.40.206" before! You can also make a personal user page once you are logged in. The link to your user page will appear at the top right of the screen. You will also have a personal talk page, and will be notified with a banner each time someone adds a message. Being able to communicate easily makes it a lot easier to build a wiki.
Being logged in allows you to create pages, rename pages (with the "move" tab), and upload images. You can also easily view your contributions using the "contributions" link at the top of the screen, and make a personal watchlist to keep an eye on your favorite articles.
A lot of the look of the site can be changed with customization. Using the "preferences" link, you can change the overall look of the site, your time-zone preferences, search settings, and much more. If you know CSS, then you can change the look of the site even further with your own personal style sheet.