If you use a normal contact form then spammers will fond a way to automatically send junk messages through that form, so you need a way to verify that the sender is an actual person and not an automated script.
A good solution for this is to use an random code that is generated by a script the code will be displayed as an image so only a person could read the image and type the number. This practice is called Captcha image validation. The image also has a noise added to avoid any existent automated scripts that could read the text from the image.
The contact form also makes sure the user filled all the form fields correctly, when all fields are completed and the text entered inside email box is an email address.
