For a while now I've noticed some websites using those pictures with 4 or 5 numbers/digits, which in addition to your user/pw, you have to fill into another box to get access.

I think it's well known from free mail services and the like to prevent robots from signing up, but what is the reason for using it for membership access?

Is it a real additional security feature that stops robots from brute force attacks or is it just another gimmick that is trendy at the moment and something you really have to have nowadays?