For reasons best known to themselves (and their psychiatrists) some people enjoy trying to mess up other people's websites.
They may, for example, write a program to automatically fill in our membership form. In a few minutes they could create millions of new
members and thus jam up our server. The numbers on the screen are easy for you to read but much harder for a program, we can be fairly confident that anyone who keys in the correct number is human - or at least fairly close to it, and that's good enough for us!