All this does is simplify the captcha for machines to solve, whilst complicating it for humans (basic maths is easy for most but it's still one more logical step then 'type out the 5 letters you see'). Instead of 26 upper and lower-case characters, and often just a random string (so we can't make a best guess vs a dictionary), we now just have ten characters and maybe 2-4 mathematical operators and it must make sense (it can't be "42+-" for example).