I understand the purpose of Captcha to show you’re human.
But, there are many ways they test for that. Another common one is asking you to retype letters and numbers and are all skewwhiff.
The ones that ask you to click on pictures I’m told are harvesting data for recognition functions for self-driving vehicles. Captcha can perform two functions at once: preventing access and creating a huge data set that is useful for something else.
Think about it: those vehicles have to recognise all kinds of things on the road. Getting lots of data about what various objects look like to humans is super helpful.
ETA: I googled it just in case I was told a tall tale. If you google it, you will see tons of confirmation that Google itself uses captcha for its self-driving vehicles.
I was not saying what the purpose of captcha is (as you rightly IDed: access); I was providing what I thought was a cool extra tidbit about it. I should have said ‘are also used to gather data’.