Why do some sites use another URL to authenticate?

I was wondering why some sites like Google or AWS use another URL to authenticate users. For instance, if I try to access Google Drive, I'm redirected to https://accounts.google.com/signin/ and after login, back to https://drive.google.com/. The same happens with AWS, which uses https://signin.aws.amazon.com to sign in and redirects to https://console.aws.amazon.com/.

As web developer, I see one advantage using such strategy which is split my app in two to decrease the main app bundle size, but I cannot see another one.

My questions:

Why do some sites use another URL to authenticate?

What are the pros and cons using this method?