The idea is that we, people, can read those things (addmitted, the ones hubpages use are not the clearest i have ever seen) and that bots that crawl the internet to post spam cannot. Thus, this feature prevents bots from posting hubs with spam on it.
However, as soon as it becomes illegible by people, as regularly happens (with me, at least), they become annoying.
I guess the words that you need to read have to become more unclear, as computers get better and better at reading, especially if the letters are clear and regular. Nowadays some systems are even able to read handwritten texts, and taking into account some people's handwritings, that is quite impressive.
In the future, these captcha systems will probably be outdated, but for now, they will just be unclear. I wonder what they will think of in the future to prevent bots from posting spam.
@Jeanieinabottle: Maybe it is the other way around; your work pc is less secure, so you have to go through an additional 'screening' to decide whether you are human or not..
I'm not sure whether this is actually the case, but it seems more plausible to me.