Security codes on blog comments

What is it about computer generated codes these days. We have to have codes for everything we do nowadays.
Even on here if we want to leave a comment on someones post we get given a code to enter before being able to either post it or submit it for moderation.
The designers of the site want to make sure that a computer isn't writing the comments! How many people would go to all the effort of programming a computer to leave comments on either their blogs, or random peoples blogs.

And then the codes they give us are at times barely legible. Now they have taken to adding numbers for us to enter too. I was given one the other day that was so blurry that i could only just read it. Now i know i wear glasses, but with them on i do have pretty good vision.

Is the chance of computer generated comments on these sites that high to now need even more codes to ensure we are human, or are the designers just making us do extra work. I really can't fathom it out.