I am not a robot

It is known as a "denial of service" attack - if it succeeds, the contact that server is providing would be unavailable to all its subscribers and any websites based on it would crash.

If you would like to know more:

en.wikipedia.org/wiki/Internet_bot

It is to prevent the 'bots doing an auto-fill (by the multi-thousand) on such pages.
If these pages did not have a "proven human reaction" box, their servers could potentially be brought down by the mega use by bots.

The "I am not a robot" ones are easier to use than the Captcha ones. Some of those are impossible to decipher so you end up having to start again.

The point of it really is to prevent the form being "filled in" or intercepted by a robot - there are roaming "bots" on the net which are on the lookout for forms like these, to try to infiltrate yours or Gransnet's systems.

All this unnecessary nonsense uses up more juice than you anticipated making them the winners in this time-consuming crap which is usually on a loop ( repeated ) .