What is the purpose of asking 'I'm not a robot": Human verification need.

To make sure you are not a robot. It’s just what it says.

Bots game the system of many websites yet they cannot see the human verification to enter. So, by asking you to verify you’re not a robot, you are proving that you aren’t going to be a bot scamming the system since you can see the request before entering.

These questions are actually a screening device. Aliens from other worlds, intent on domination of earth, want to ensure that they are identifying human life forms as opposed to animal so that when their conquest begins, they won’t waste time subduing anything but humans.

By answering these Captcha (or “I am a human”) tests, you are essentially identifying yourself as a potential target for extermination by our alien overlords.

Bots can open many accounts in the blink of an eye. Nefarious users can use these accounts to send spam, or perform denial of service attacks, or participate in other low-life activities.
Proving you are not a robot slows them down.
Look at this place and how often we are spammed by human bots sitting in a sweat shop in Hyderabad opening accounts. Imagine if they were able to let bots do it automatically.

This is a basic step taken by many websites to avoid bot-comments/robot spammers from posting rubbish on their websites