[rum3]

Since they introduced the square-selecting captchas I have always assumed that they use it for identifying the user. I bet that depending on how you solve the captchas they can identify who you are if their system already has a theory of who you might be.

[kmlx]

they're there for denying access to automated scripts.

[pawelk]

This is the reason they exist in the first place, but doesn't answer the question why they're implemented this particular way.