Make money
one of the more obvious reasons is to make money, you can start a business, a blog or whatever interests you, you can later change it to suit you better.
Showcase you cv
You can have a website to showcase your cv, instead of sending people your cv in only text, send it to them as a url as well. That will impress them more, especially if you are a graphic designer. I have seen some of the most impresive cv's online in the form of a websites. Some payed blog jobs only look at your blog to give you the job. For webdesigners you dont even need a cv all you need is a website.
Express yourself
With you own website you can start a blog and type what you want, some social networking sites and some blog hosting places wont let you type certain things. For example express yourself politically with no filtering.
Use your website as your central place on the internet.
With so many widgets to choose from and rss feeds you can turn you own website into a brilliant start page directly to the links and things you use daily.
You can also embed more than one search engine at a time and make the site like you want it. You can even use iframes since you are not building the site for other people or search engines but for yourself.
Keep the domain for later.
You can always look for a great domain name and park it even if you dont use it. Later on when all the domains are taken in your country you can sell it for a lot of money.
Stay in touch with friends.
A lot of social networking sites allows you to feed your profile and comments. Why not intergrate it into your own website.
Use it as a business card
Put all your contact details on there and only give people your url to see everything they need to know about you or your business.
yea nice Work :D