Because it lets you use it as a geographically distributed cache basically. Most GET requests should be able to be cachable on the typical website, besides the ones that are unique per logged in user or similar.

If your server is in Europe, and you have Australian users, there is a physical limit how low the response times can go, serving bits over that distance, so only way you can make it go below that limit, is by moving where you serve the data from closer to the user.

Lots of websites have horrible performance for whatever reasons, and lots of freelancers/consultants basically default to throwing a CDN on top of those when they get approached by businesses to fix the slow website browsing, as they're not the ones who have to pay the monthly subscription, and the less work they have to do, the better $ per hour spent for them.

I agree that it's a shit solution and there is much better sustainable ways of solving these things.

Your webserver most likely have "rate limiting" built in already, which you can configure to act based on lots of variables typically. Set a limit of 1 req/s or whatever, and you've stopped 99% of all DDoS you'll encounter on the public web. If your visitors get cranky, up it to 10 req/s and you still are preventing most of the "abusive traffic", granted your backend/website isn't completely upside down when it comes to performance and resource usage.

CDN is something you do once you run out of options, not something you should reach for immediately, it makes no sense in most cases of just hosting a website.