There's a split debate as to whether it's needed or not.
Some say it is, some say it isn't.
Those who say it isn't needed, will argue that if you only put a small web sized preview of a shot up, then there's no need to watermark it as it will be too small for print use.
However, we had a member on here who only 2 weeks ago, had a photo stolen from his website and used on another one. I have heard similar instances around the internet too.
Another one on here had his photo used after a site in question cropped off his copyright notice from the bottom!
Those who say it is needed will argue that it's to protect their intellectual property. Naturally if you sell your images, then it might be considered a financial security, just incase.
It's all down to personal preference though. Some people would just be flattered if their images are taken from the website and used elsewhere, while others would be astonished and feel stolen from.
Both are equally valid thoughts and opinions though, so really it is down to personal choice.