If you run a website, then by default it is accessible to the whole planet. Many websites are simply not relevant to people in other countries. So, you should not expect significant traffic from them as a matter of course.
If you have a local bookstore and your primary market is local people walking into your store, then there is no need to let any other countries index or waste bandwidth on your server. The same might be true of a carwash, or babysitters, or lawn mowing.
Below are many of the common ways to block countries, with some pros and cons and code samples.
2. Use a Hosting Company That Has Blocking as Part of Their Built-in Controls
3. Content Delivery Networks