The robots are programs designed to crawl sites for various reasons. They are easy to create by programmers. Not having CAPTCHA can result in:
– content theft
– credentials theft via brute force attack
– being scanned for vulnerabilities
– increased server load (a script instance can load a server as much as thousands of users)
In many cases CAPTCHA as devolved into a due diligence checklist item that insipid security auditors with no technical knowledge insist upon for no real reason.
If you provide an online service – something that looks something up, or calculates something – you want to stop people from abusing/overloading the system. If the users need to proof they are human, automated scripts are at least slowed somewhat, so other people can get their turn.
For free online games you want to reduce the people who are able to create new accounts and play them automatically just to give themself help or farm random items.
CAPTCHAs aren’t terribly useful for stopping concentrated attacks, but they reduce the amount of spam that is sent via publicly accessible forms.
An easy example are forums where people post replies to threads, or websites that allow comments from readers.
If there is no way to verify it is a real person, then the threads and comments quickly fill up with spam ads, links to porn, etc.
Moderators can clean them up by deleting them, but it makes their job a lot harder if the spammers can write programs to litter their ads on every page quickly.
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