It exists to identify if whoever is visiting is a human or a bot.

Why?

Because if it's a bit it's likely automated and there's a variety of reasons to not want automated requests to come through. Some of them are:

• because lots of "hackers" try to hack systems by automatically trying to find security vulnerabilities in a server, service etc.
• because automated requests can be done faster, so they can happen more commonly, and they use resources, and if there's too many this genuinely affects server performance budget
• bunch more reasons