Anti-Spam Precaution
Professional spammers use automated processes to hit guest book and inquiry forms with hundreds of spam submissions. These spam programs aren't intelligent, they just follow links, find recognizable form fields, and send useless rubbish. There are ways to prevent certain submissions, for instance by blocking any content with a URL in it, or by blocking certain IP addresses.
But by adding a simple random sum to the form, the automated programs don't work at all because the form needs a "human eye" to enter the right answer – and the right answer must be present for the form to submit.