This is quite simple. Web spiders (robots) are very hungry for your e-mails and forms. Once they get access to it, you will get a lots of bulk e-mails and your webforms (forums) will full of rubbish.

Good way of prevention of that is to use CAPTCHA challenge, which is too complicated to use for software, but easy for human. There is no need to publish our e-mail address or be afraid of order forms - you are secure. How it works you can see under in example of our contact form.